When testing compiler flags, test them with previously-tested flags.

That mirrors what we're doing with autotools, and catches cases where
an earlier-tested flag affects the results for a later-tested flag
(e.g., -fwrapv can affect whether -fno-strict-overflow produces a
warning when tested).

Change-Id: I8ca0b6f5dee14d2394af9b6b8b93b9dbda2702d7
Reviewed-on: https://code.wireshark.org/review/3369
Reviewed-by: Guy Harris <guy@alum.mit.edu>