build: get rid of "unrecognized … anon-tag"

GCC only errors out on unrecognized _positive_ `-Wsomething` flags,
negative ones it ignores... but then prints a "note" about whenever an
unrelated(!) warning/error occurs.  This is both annoying and confusing,
since we now get a lot of:

```
cc1: note: unrecognized command-line option ‘-Wno-microsoft-anon-tag’ may have been intended to silence earlier diagnostics
```

Fix by checking for the positive flag in `AC_C_FLAG` instead, which will
error out, and therefore `-Wno-microsoft-anon-tag` won't be added for
GCC.  (It only exists on clang.)

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>

diff --git a/configure.ac b/configure.ac
index f11b345cf6ef927efc19c499e5674350a7a2a8a7..6d8e70bb86b26dc6681b8c120dd3a011acb3ce0e 100644 (file)
--- a/configure.ac
+++ b/configure.ac
@@ -275,7 +275,16 @@ AC_DEFUN([AC_C_FLAG], [{
     AC_CACHE_CHECK([[whether $CC supports $1]], cachename, [
        AC_LANG_PUSH([C])
        ac_c_flag_save="$CFLAGS"
-       CFLAGS="$CFLAGS $1"
+       dnl GCC ignores unknown -Wno-whatever flags, but errors on -Wwhatever
+       dnl except when it ignores them it prints:
+       dnl   cc1: note: unrecognized command-line option ‘-Wno-whatever’ may have been intended to silence earlier diagnostics
+       dnl which is annoying as hell.  So check for the positive flag instead.
+       flag_add="$1"
+       if test "$flag_add" != "${flag_add#-Wno-}"; then
+           CFLAGS="$CFLAGS -W${flag_add#-Wno-}"
+       else
+           CFLAGS="$CFLAGS $flag_add"
+       fi
        AC_COMPILE_IFELSE(
                [AC_LANG_PROGRAM([[$4]])],
                [