Make popcount cross-platform

[eelstork]

__builtin__popcount is a built-in GCC function. There is no standard C/C++ equivalent.

For portability I provide an explicit popcount implementation; for performance on Windows, MSVC built-ins are suggested.

Compiler specific code may be avoided; should we make an exception in this case?

Implementation note

_GNUC_ is also defined with llvm/clang; testing for __llvm__ and/or __clang__ will not behave as expected. Less naive compiler macros might be used (at the cost of adding Boost headers).

Currently pop count is only used in hamming_distance; It would appear that Caffe does not use this function.

[ronghanghu]

I guess you don't need this one. You can perhaps just remove caffe_cpu_hamming_distance IMO.

Actually I don't think you need caffe_cpu_hamming_distance at all, and it is perhaps a mistake to merge it one year ago...

[longjon]

Obsoleted by #3404.