[buddy] add -Wno-gnu if supported

* m4/gccwarns.m4: Add -Wno-gnu to workaround a diagnostic from
clang 3.9.1 on arch Linux.  clang was complaining whtat assert()
is using a GNU extension.

buddy/m4/gccwarns.m4

@@ -41,6 +41,7 @@ EOF
    Wnull-dereference \
    Wsuggest-override \
    Wpedantic \
+   Wno-gnu \
    Wno-long-long
   do
     CFLAGS="$cf_save_CFLAGS $ac_cv_prog_gcc_warn_flags -$cf_opt"