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>

0 files changed, 0 insertions, 0 deletions