Michael Brown
1c47eb186e
[build] Use positive-form tests when checking for supported warnings
Some versions of gcc seem to silently accept an attempt to disable an
unrecognised warning (e.g. via -Wno-stringop-truncation) but will then
report the unrecognised warning if any other error occurs during the
build, resulting in a potentially misleading error message.
Avoid this potential confusion by using the positive-form tests in
order to determine the workaround CFLAGS.
Signed-off-by: Michael Brown <mcb30@ipxe.org>