gtk/gtkcssenumvalue.c: Deal with __builtin_popcount on MSVC

gtk/gtkcssenumvalue.c: Deal with __builtin_popcount on MSVC

__builtin_popcount is a GCCism that is used to count the number of bits
involved, which means any non GCC/CLang compilers won't like the code,
meaning that on MSVC builds we must implement it ourselves.

We first use __cpuid() to check whether the CPU supports the popcount
instruction, if it does, we use the __popcnt intrinsic, otherwise
(untested, since I don't have a system that does not have the
instruction), we use the suggested hacks at
http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel

https://bugzilla.gnome.org/show_bug.cgi?id=773299