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 971d136..740c455 100644
--- a/gettext-tools/libgettextpo/Makefile.am
+++ b/gettext-tools/libgettextpo/Makefile.am
@@ -42,7 +42,7 @@ AM_CPPFLAGS = \
   -I../src -I$(top_srcdir)/src \
   -I../../gettext-runtime/intl -I$(top_srcdir)/../gettext-runtime/intl

-DEFS = -DIN_LIBGETTEXTPO=1 -DOMIT_SETLOCALE_LOCK=1 @DEFS@
+DEFS = -DIN_LIBGETTEXTPO=1 -DOMIT_SETLOCALE_LOCK=0 @DEFS@

 # Enable more warning options in this directory.
 AM_CFLAGS = @WARN_CFLAGS@
diff --git a/gettext-tools/libgettextpo/Makefile.in b/gettext-tools/libgettextpo/Makefile.in
index 0e655ea..57bb2c5 100644
--- a/gettext-tools/libgettextpo/Makefile.in
+++ b/gettext-tools/libgettextpo/Makefile.in
@@ -1376,7 +1376,7 @@ CXX_HAS_CHAR8_TYPE = @CXX_HAS_CHAR8_TYPE@
 CXX_HAS_UCHAR_TYPES = @CXX_HAS_UCHAR_TYPES@
 CYGPATH_W = @CYGPATH_W@
 DC = @DC@
-DEFS = -DIN_LIBGETTEXTPO=1 -DOMIT_SETLOCALE_LOCK=1 @DEFS@
+DEFS = -DIN_LIBGETTEXTPO=1 -DOMIT_SETLOCALE_LOCK=0 @DEFS@
 DEPDIR = @DEPDIR@
 DESIRED_LINGUAS = @DESIRED_LINGUAS@
 DFLAGS = @DFLAGS@
-- 
2.35.1