Improve compiler detection for __builtin_popcount()

Improve compiler detection for __builtin_popcount()

The popcount builtin was added in GCC after version 4.2 (which is what
some *BSDs are using), which means we need to be more specific when
using it than just asking for GCC.

While we're at it, we can improve the compiler detection, and use a
builtin popcount on Clang ≥ 3.1 and MSVC 2008.

https://bugzilla.gnome.org/show_bug.cgi?id=755455