configure.ac: disable FORTIFY_SOURCE for asan/msan/tsan

This can cause trouble with these specific sanitizers according to https://github.com/google/sanitizers/issues/247#issuecomment-242238980 . The MSAN CI build already disables hardening, as there can be false positives there due to lack of instrumentation on the fortified functions (e.g. __memcpy_chk), but there's no reason the other hardening checks (stack-smashing protection, and more) can't be enabled if this is the culprit. Also affects ASAN and TSAN, though I do not know how as I only encountered a problem with MSAN. This patch will disable source fortification when --with-sanitizers is used with these sanitizers, but if -fsanitize is used instead, then this patch will be bypassed.