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.

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 <>
Reviewed-by: Julius Werner <>
Tested-by: Patrick Georgi <>
Commit-Queue: Patrick Georgi <>
1 file changed