Use pkg-config instead of brittle AC_CHECK_LIB hackery

--- a/configure.ac
+++ b/configure.ac
@@ -142,41 +142,11 @@
 ] )
 
 # Check for libpng
-AC_CHECK_LIB(png, png_create_read_struct, [
-AC_SUBST(PNG_LIBS, "-lpng")
-], [
-  AC_CHECK_LIB(png10, png_create_read_struct, [
-  AC_SUBST(PNG_LIBS, "-lpng10")
-  ], [
-    AC_CHECK_LIB(png12, png_create_read_struct, [
-    AC_SUBST(PNG_LIBS, "-lpng12")
-    ], [
-      AC_MSG_ERROR([cannot find required library png])
-    ], [])
-  ], [])
-], [])
-AC_CHECK_HEADERS([png.h libpng/png.h libpng10/png.h libpng12/png.h])
+PKG_CHECK_MODULES([PNG], [libpng])
 
 # Check for libopenjpeg, fall back to libjasper if not available
-AC_CHECK_LIB(jasper, jas_init, [
-AC_SUBST(JP2000_LIBS, "-ljasper")
-AC_CHECK_HEADERS([jasper/jasper.h])
-AC_DEFINE([ICNS_JASPER],[1],[We have Jasper])
-], [
-  AC_CHECK_LIB(openjpeg2, opj_setup_decoder, [
-  AC_SUBST(JP2000_LIBS, "-lopenjpeg2")
-  AC_CHECK_HEADERS([openjpeg.h])
-  AC_DEFINE([ICNS_OPENJPEG],[1],[We have OpenJPEG])
-  ], [
-    AC_CHECK_LIB(openjpeg, opj_setup_decoder, [
-    AC_SUBST(JP2000_LIBS, "-lopenjpeg")
-    AC_CHECK_HEADERS([openjpeg.h])
-    AC_DEFINE([ICNS_OPENJPEG],[1],[We have OpenJPEG])
-    ], [
-      AC_MSG_WARN([libopenjpeg or libjasper jp2000 codec libraries not found])
-      AC_MSG_WARN([libicns will be built without 256x256 and 512x512 support])
-    ])
-  ])
+PKG_CHECK_MODULES([JP2000], [libopenjp2], [
+  AC_DEFINE([ICNS_OPENJPEG], [2], [We have OpenJPEG 2.x])
 ])
 
 AC_CONFIG_FILES([Makefile libicns.spec icnsutils/Makefile src/Makefile src/libicns.pc])
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -3,7 +3,8 @@
 
 libicns_la_LDFLAGS = -version-info 3:0:2
 
-libicns_la_LIBADD = @PNG_LIBS@ @JP2000_LIBS@
+libicns_la_LIBADD = $(PNG_LIBS) $(JP2000_LIBS)
+libicns_la_CPPFLAGS = $(PNG_CFLAGS) $(JP2000_CFLAGS)
 
 libicns_la_SOURCES = \
   icns_debug.c \