OMIT_SETLOCALE_LOCK should be true when gl_get_setlocale_null_lock is provided by a dependency library, like GNU libintl.
On Gentoo musl we use musl's libintl and gl_get_setlocale_null_lock is not provided by it, therefore we set OMIT_SETLOCALE_LOCK to 0.

To properly fix this we should do AC_SEARCH_LIBS([gl_get_setlocale_null_lock], [intl], ...) in automake, but instead we do this 
hack to not get complacent. This should be fixed upstream.

https://savannah.gnu.org/bugs/index.php?62659#comment7
https://bugs.gentoo.org/830590
diff --git a/gettext-tools/libgettextpo/Makefile.am b/gettext-tools/libgettextpo/Makefile.am
index 59b016e..d92978d 100644
--- a/gettext-tools/libgettextpo/Makefile.am
+++ b/gettext-tools/libgettextpo/Makefile.am
@@ -40,7 +40,7 @@ AM_CPPFLAGS = \
   -I../src -I$(top_srcdir)/src \
   -I../intl -I$(top_srcdir)/../gettext-runtime/intl
 
-DEFS = -DIN_LIBGETTEXTPO=1 -DOMIT_SETLOCALE_LOCK=1 @DEFS@
+DEFS = -DIN_LIBGETTEXTPO=1 -DOMIT_SETLOCALE_LOCK=0 @DEFS@
 
 # libgettextpo contains the public API for PO files.
 libgettextpo_la_SOURCES = \
diff --git a/gettext-tools/libgettextpo/Makefile.in b/gettext-tools/libgettextpo/Makefile.in
index f76efa5..aca3a81 100644
--- a/gettext-tools/libgettextpo/Makefile.in
+++ b/gettext-tools/libgettextpo/Makefile.in
@@ -804,7 +804,7 @@ CXXDEPMODE = @CXXDEPMODE@
 CXXFLAGS = @CXXFLAGS@
 CXX_CHOICE = @CXX_CHOICE@
 CYGPATH_W = @CYGPATH_W@
-DEFS = -DIN_LIBGETTEXTPO=1 -DOMIT_SETLOCALE_LOCK=1 @DEFS@
+DEFS = -DIN_LIBGETTEXTPO=1 -DOMIT_SETLOCALE_LOCK=0 @DEFS@
 DEPDIR = @DEPDIR@
 DLLTOOL = @DLLTOOL@
 DSYMUTIL = @DSYMUTIL@
-- 
2.35.1