Enforce stack protector usage

If --enable-stack-protector is used is used, configure script try to use
--fstack-protector-strong. In case it's not supported, --fstack-protector-all
is enabled. If both protectors are not supported, configure does not use
any protector at all without any notification.

This patch reports error when user requests stack protector to be used and
both protector modes are not supported. Behavior is not changed in case
user do not use any of --enable-stack-protector/--disable-stack-protector.

Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
[Fix non-POSIX operator in test. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
