configure: use do_cc when checking for -fstack-protector support

MacOSX clang silently swallows unrecognized -f options when doing a link
with '-framework' also on the command line, so to detect support for
the various -fstack-protector options we must do a plain .c to .o compile,
not a complete compile-and-link.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-id: 1397041487-28477-1-git-send-email-peter.maydell@linaro.org
