meson: avoid checking compiler flags twice

meson: avoid checking compiler flags twice

In several cases we check if a compiler flag is supported, and then add
it to the 'cc_flags' array. The entire 'cc_flags' array is then later
tested to see if each flag is supported, which duplicates the check in
some cases.

Move the check of cc_flags earlier, and for the extra flags append
directly to supported_cc_flags to avoid the duplicate check

Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>