Make win32 builds always do SetEnvironmentVariable() when doing putenv().

Also, if linked against other versions than the default MSVCRT library
(for example the MSVC build which links against MSVCRT80), also update
the cache in the default MSVCRT at the same time.

This should fix the issues with setting LC_MESSAGES on the MSVC build.

Original patch from Hiroshi Inoue and Hiroshi Saito, much rewritten
by me.