Improve static checking of integer overflow and stack smashing.

Improve static checking of integer overflow and stack smashing.

* configure.ac (WARN_CFLAGS): Add -Wstack-protector and
-Wstrict-overflow if using GCC 4.7.2 or later on a platform with
at least 64-bit long int.  This improves static checking on these
platforms, when configured with --enable-gcc-warnings.