Let GOOGLE_COMPILE_ASSERT use static_assert if available.

The motivation is that gcc 4.8+ and clang trunk warn on unused local
typedefs, which COMPILE_ASSERT adds. After this change, the warning
will be happy at least in C++11 builds. static_assert also produces a
slighly nicer diagnostic than the typedef method.

https://github.com/google/re2/commit/eb93e8bc43ac8d05322fb3e9fc885898ad924f8a
did the same change in re2.