check cc flags viability before setting them
diff --git a/aclocal.m4 b/aclocal.m4
index 05fefc2..ff520db 100644
--- a/aclocal.m4
+++ b/aclocal.m4
@@ -970,6 +970,7 @@
 AC_SUBST([am__untar])
 ]) # _AM_PROG_TAR
 
+m4_include([m4/ax_check_compile_flag.m4])
 m4_include([m4/libtool.m4])
 m4_include([m4/ltoptions.m4])
 m4_include([m4/ltsugar.m4])