https://bugs.gentoo.org/818775

--- a/Makefile.am
+++ b/Makefile.am
@@ -111,7 +111,7 @@ protoc_c_protoc_gen_c_LDADD = \
 	-lprotoc
 
 protobuf-c/protobuf-c.pb.cc protobuf-c/protobuf-c.pb.h: @PROTOC@ $(top_srcdir)/protobuf-c/protobuf-c.proto
-	$(AM_V_GEN)@PROTOC@ -I$(top_srcdir) --cpp_out=$(top_builddir) $(top_srcdir)/protobuf-c/protobuf-c.proto
+	$(AM_V_GEN)@PROTOC@ -I$(top_srcdir) -I@includedir@ --cpp_out=$(top_builddir) $(top_srcdir)/protobuf-c/protobuf-c.proto
 
 BUILT_SOURCES += \
 		 protobuf-c/protobuf-c.pb.cc \
@@ -174,16 +174,16 @@ t_generated_code2_cxx_generate_packed_da
 	$(protobuf_LIBS)
 
 t/test.pb-c.c t/test.pb-c.h: $(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) $(top_srcdir)/t/test.proto
-	$(AM_V_GEN)@PROTOC@ --plugin=protoc-gen-c=$(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) -I$(top_srcdir) --c_out=$(top_builddir) $(top_srcdir)/t/test.proto
+	$(AM_V_GEN)@PROTOC@ --plugin=protoc-gen-c=$(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) -I$(top_srcdir) -I@includedir@ --c_out=$(top_builddir) $(top_srcdir)/t/test.proto
 
 t/test-optimized.pb-c.c t/test-optimized.pb-c.h: $(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) $(top_srcdir)/t/test-optimized.proto
-	$(AM_V_GEN)@PROTOC@ --plugin=protoc-gen-c=$(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) -I$(top_srcdir) --c_out=$(top_builddir) $(top_srcdir)/t/test-optimized.proto
+	$(AM_V_GEN)@PROTOC@ --plugin=protoc-gen-c=$(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) -I$(top_srcdir) -I@includedir@ --c_out=$(top_builddir) $(top_srcdir)/t/test-optimized.proto
 
 t/test-full.pb-c.c t/test-full.pb-c.h: $(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) $(top_srcdir)/t/test-full.proto
-	$(AM_V_GEN)@PROTOC@ --plugin=protoc-gen-c=$(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) -I$(top_srcdir) --c_out=$(top_builddir) $(top_srcdir)/t/test-full.proto
+	$(AM_V_GEN)@PROTOC@ --plugin=protoc-gen-c=$(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) -I$(top_srcdir) -I@includedir@ --c_out=$(top_builddir) $(top_srcdir)/t/test-full.proto
 
 t/test-full.pb.cc t/test-full.pb.h: @PROTOC@ $(top_srcdir)/t/test-full.proto
-	$(AM_V_GEN)@PROTOC@ -I$(top_srcdir) --cpp_out=$(top_builddir) $(top_srcdir)/t/test-full.proto
+	$(AM_V_GEN)@PROTOC@ -I$(top_srcdir) -I@includedir@ --cpp_out=$(top_builddir) $(top_srcdir)/t/test-full.proto
 
 t/generated-code2/test-full-cxx-output.inc: t/generated-code2/cxx-generate-packed-data$(EXEEXT)
 	$(AM_V_GEN)$(top_builddir)/t/generated-code2/cxx-generate-packed-data$(EXEEXT) > $(top_builddir)/t/generated-code2/test-full-cxx-output.inc
@@ -214,7 +214,7 @@ t_generated_code3_test_generated_code3_S
 	t/test-proto3.pb-c.c
 
 t/test-proto3.pb-c.c t/test-proto3.pb-c.h: $(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) $(top_srcdir)/t/test-proto3.proto
-	$(AM_V_GEN)@PROTOC@ --plugin=protoc-gen-c=$(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) -I$(top_srcdir) --c_out=$(top_builddir) $(top_srcdir)/t/test-proto3.proto
+	$(AM_V_GEN)@PROTOC@ --plugin=protoc-gen-c=$(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) -I$(top_srcdir) -I@includedir@ --c_out=$(top_builddir) $(top_srcdir)/t/test-proto3.proto
 
 BUILT_SOURCES += \
 	t/test-proto3.pb-c.c t/test-proto3.pb-c.h
@@ -237,7 +237,7 @@ t_issue204_issue204_SOURCES = \
 t_issue204_issue204_LDADD = \
 	protobuf-c/libprotobuf-c.la
 t/issue204/issue204.pb-c.c t/issue204/issue204.pb-c.h: $(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) $(top_srcdir)/t/issue204/issue204.proto
-	$(AM_V_GEN)@PROTOC@ --plugin=protoc-gen-c=$(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) -I$(top_srcdir) --c_out=$(top_builddir) $(top_srcdir)/t/issue204/issue204.proto
+	$(AM_V_GEN)@PROTOC@ --plugin=protoc-gen-c=$(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) -I$(top_srcdir) -I@includedir@ --c_out=$(top_builddir) $(top_srcdir)/t/issue204/issue204.proto
 BUILT_SOURCES += \
 	t/issue204/issue204.pb-c.c t/issue204/issue204.pb-c.h
 EXTRA_DIST += \
@@ -254,7 +254,7 @@ t_issue220_issue220_SOURCES = \
 t_issue220_issue220_LDADD = \
 	protobuf-c/libprotobuf-c.la
 t/issue220/issue220.pb-c.c t/issue220/issue220.pb-c.h: $(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) $(top_srcdir)/t/issue220/issue220.proto
-	$(AM_V_GEN)@PROTOC@ --plugin=protoc-gen-c=$(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) -I$(top_srcdir) --c_out=$(top_builddir) $(top_srcdir)/t/issue220/issue220.proto
+	$(AM_V_GEN)@PROTOC@ --plugin=protoc-gen-c=$(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) -I$(top_srcdir) -I@includedir@ --c_out=$(top_builddir) $(top_srcdir)/t/issue220/issue220.proto
 BUILT_SOURCES += \
 	t/issue220/issue220.pb-c.c t/issue220/issue220.pb-c.h
 EXTRA_DIST += \
@@ -271,7 +271,7 @@ t_issue251_issue251_SOURCES = \
 t_issue251_issue251_LDADD = \
 	protobuf-c/libprotobuf-c.la
 t/issue251/issue251.pb-c.c t/issue251/issue251.pb-c.h: $(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) $(top_srcdir)/t/issue251/issue251.proto
-	$(AM_V_GEN)@PROTOC@ --plugin=protoc-gen-c=$(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) -I$(top_srcdir) --c_out=$(top_builddir) $(top_srcdir)/t/issue251/issue251.proto
+	$(AM_V_GEN)@PROTOC@ --plugin=protoc-gen-c=$(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) -I$(top_srcdir) -I@includedir@ --c_out=$(top_builddir) $(top_srcdir)/t/issue251/issue251.proto
 BUILT_SOURCES += \
 	t/issue251/issue251.pb-c.c t/issue251/issue251.pb-c.h
 EXTRA_DIST += \
@@ -289,14 +289,14 @@ t_issue330_issue330_SOURCES = \
 t_issue330_issue330_LDADD = \
 	protobuf-c/libprotobuf-c.la
 t/issue330/issue330.pb-c.c t/issue330/issue330.pb-c.h: $(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) $(top_srcdir)/t/issue330/issue330.proto
-	$(AM_V_GEN)@PROTOC@ --plugin=protoc-gen-c=$(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) -I$(top_srcdir) --c_out=$(top_builddir) $(top_srcdir)/t/issue330/issue330.proto
+	$(AM_V_GEN)@PROTOC@ --plugin=protoc-gen-c=$(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) -I$(top_srcdir) -I@includedir@ --c_out=$(top_builddir) $(top_srcdir)/t/issue330/issue330.proto
 BUILT_SOURCES += \
 	t/issue330/issue330.pb-c.c t/issue330/issue330.pb-c.h
 
 t_issue330_issue330_SOURCES += \
 	t/issue389/issue389.pb-c.c # Tack onto issue330 since there is no need for a separate binary here
 t/issue389/issue389.pb-c.c t/issue389/issue389.pb-c.h: $(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) $(top_srcdir)/t/issue389/issue389.proto
-	$(AM_V_GEN)@PROTOC@ --plugin=protoc-gen-c=$(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) -I$(top_srcdir) --c_out=$(top_builddir) $(top_srcdir)/t/issue389/issue389.proto
+	$(AM_V_GEN)@PROTOC@ --plugin=protoc-gen-c=$(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) -I$(top_srcdir) -I@includedir@ --c_out=$(top_builddir) $(top_srcdir)/t/issue389/issue389.proto
 BUILT_SOURCES += \
 	t/issue389/issue389.pb-c.c t/issue389/issue389.pb-c.h
 EXTRA_DIST += \
@@ -312,7 +312,7 @@ t_issue440_issue440_SOURCES = \
 t_issue440_issue440_LDADD = \
 	protobuf-c/libprotobuf-c.la
 t/issue440/issue440.pb-c.c t/issue440/issue440.pb-c.h: $(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) $(top_srcdir)/t/issue440/issue440.proto
-	$(AM_V_GEN)@PROTOC@ --plugin=protoc-gen-c=$(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) -I$(top_srcdir) --c_out=$(top_builddir) $(top_srcdir)/t/issue440/issue440.proto
+	$(AM_V_GEN)@PROTOC@ --plugin=protoc-gen-c=$(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) -I$(top_srcdir) -I@includedir@ --c_out=$(top_builddir) $(top_srcdir)/t/issue440/issue440.proto
 BUILT_SOURCES += \
 	t/issue440/issue440.pb-c.c t/issue440/issue440.pb-c.h
 EXTRA_DIST += \
@@ -332,7 +332,7 @@ t_issue375_issue375_SOURCES = \
 t_issue375_issue375_LDADD = \
 	protobuf-c/libprotobuf-c.la
 t/issue375/issue375.pb-c.c t/issue375/issue375.pb-c.h: $(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) $(top_srcdir)/t/issue375/issue375.proto
-	$(AM_V_GEN)@PROTOC@ --plugin=protoc-gen-c=$(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) -I$(top_srcdir) --c_out=$(top_builddir) $(top_srcdir)/t/issue375/issue375.proto
+	$(AM_V_GEN)@PROTOC@ --plugin=protoc-gen-c=$(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) -I$(top_srcdir) -I@includedir@ --c_out=$(top_builddir) $(top_srcdir)/t/issue375/issue375.proto
 BUILT_SOURCES += \
 	t/issue375/issue375.pb-c.c t/issue375/issue375.pb-c.h
 EXTRA_DIST += \