Fix warning from m4/attributes.m4

Another AC_LANG_SOURCE warning that wasn't fixed yet.

Author: xyzzy42

m4/attributes.m4

@@ -70,7 +70,7 @@ AC_DEFUN([CC_CHECK_LDFLAGS], [
     AS_TR_SH([cc_cv_ldflags_$1]),
     [ac_save_LDFLAGS="$LDFLAGS"
      LDFLAGS="$LDFLAGS $1"
-     AC_LINK_IFELSE([int main() { return 1; }],
+     AC_LINK_IFELSE([AC_LANG_SOURCE([int main() { return 1; }])],
        [eval "AS_TR_SH([cc_cv_ldflags_$1])='yes'"],
        [eval "AS_TR_SH([cc_cv_ldflags_$1])="])
      LDFLAGS="$ac_save_LDFLAGS"