[makefile] Fix -fno-stack-protector test on older versions of gcc

[makefile] Fix -fno-stack-protector test on older versions of gcc

Some older versions of gcc don't complain about unknown compiler flags
unless you ask them to actually compile; asking them to merely
preprocess won't trigger the error.

Fix the -fno-stack-protector test by making it attempt to compile an
empty file, rather than preprocess an empty file.