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

[eqvinox]

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.)