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 <d4772d05997b8abf035041e3b4f4996380ea7e7a@redhat.com>
Signed-off-by: Daniel P. Berrangé <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

crobinso/libvirt,olafhering/libvirt,crobinso/libvirt,libvirt/libvirt,zippy2/libvirt,libvirt/libvirt,nertpinx/libvirt,olafhering/libvirt,jfehlig/libvirt,crobinso/libvirt,zippy2/libvirt,zippy2/libvirt,libvirt/libvirt,libvirt/libvirt,crobinso/libvirt,jfehlig/libvirt,jfehlig/libvirt,nertpinx/libvirt,nertpinx/libvirt,jfehlig/libvirt,nertpinx/libvirt,zippy2/libvirt,olafhering/libvirt,nertpinx/libvirt,olafhering/libvirt