make/os_win sets BIT variable to 64 always

[syclik]

This doesn't run on Mac. We should either set this conditionally or leave it off.

[wds15]

Hi!

In SHA: 82b9fe6 I am propsing a fix to the problem. This patch sets the BIT variable to 32 on 32bit machines and to 64 on 64bit capable windows OS.

The logic is taken from

http://blogs.msdn.com/b/david.wang/archive/2006/03/26/howto-detect-process-bitness.aspx
and
https://msdn.microsoft.com/en-us/library/aa384274(VS.85).aspx

The thing is that on 64 bit windows machines the variable PROCESSOR_ARCHITECTURE may be set to x86 as for a 32 bit machine, but this is due to the fact that the process is a 32 bit process (32bit process run on 64bit windows). However, only when a 32 bit process runs on a 64bit machine the extra variable PROCESSOR_ARCHITEW6432 is set.

I did test the change on win32 and it works correctly, I don't have a 64bit windows available - could someone with access to 64bit windows test that -m64 appears when compiling with g++? Thanks - then we can trigger the pull request.