Always check some -f flags.

Now that we check each flag with all previously-accepted flags, this
catches the case where -fwrapv causes -fno-strict-overflow to produce a
warning when tested.

This way, what we do matches what the autotools do.

Change-Id: I68ce281dc33a32b591e3137db626f2749c4a0708
Reviewed-on: https://code.wireshark.org/review/3370
Reviewed-by: Guy Harris <guy@alum.mit.edu>