Makefile: Test for warning flags before using them

Test for warning flags that older gcc versions don't support
and only use them if supported.

BUG=none
TEST=vboot builds with gcc 4.9, ensured with manual tests that the
test_ccflag operator works correctly.

Change-Id: I14c8cbe9a687981f195d481f744db12d8877a3e0
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2550799
Reviewed-by: Julius Werner <jwerner@chromium.org>
Tested-by: Patrick Georgi <pgeorgi@chromium.org>
Commit-Queue: Patrick Georgi <pgeorgi@chromium.org>