https://bugs.gentoo.org/429422
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -6,4 +6,4 @@
 		boss.c boss.h
 
-CFLAGS = @X_CFLAGS@ @KXL_CFLAGS@
+AM_CFLAGS = @X_CFLAGS@ @KXL_CFLAGS@
 LIBS = @X_LIBS@ -lX11 @KXL_LIBS@ -lKXL