Bug: https://bugs.gentoo.org/895100
--- a/Makefile.in
+++ b/Makefile.in
@@ -65,7 +65,7 @@ EXTRA_CPPFLAGS	= @EXTRA_CPPFLAGS@
 EXTRA_LOADFLAGS	= @IMAKE_LOADFLAGS@
 
 PIXMAPDIR_DEF	= @no_pixmapdir@-DPIXMAP_ROOTDIR=\"@PIXMAPDIR@/\"
-CPPFLAGS	= -I. -I$(srcdir) -DHAVE_CONFIG_H @CPPFLAGS@ -DDEFCLASS=\"@APP_CLASS@\" $(PIXMAPDIR_DEF) $(EXTRA_CPPFLAGS)
+CPPFLAGS	= -I. -I$(srcdir) -DHAVE_CONFIG_H -D_GNU_SOURCE @CPPFLAGS@ -DDEFCLASS=\"@APP_CLASS@\" $(PIXMAPDIR_DEF) $(EXTRA_CPPFLAGS)
 CFLAGS		= @CFLAGS@ $(EXTRA_CFLAGS)
 LDFLAGS		= @LDFLAGS@ @EXTRA_LDFLAGS@
 LIBS		= @LIBS@