use cdecl for hashsig sorting functions on Windows
fixing some warnings on Windows
Add GIT_STDLIB_CALL

This removes the one-off GIT_CDECL and adds a new standard way of
doing this named GIT_STDLIB_CALL with a src/win32 specific def
when on the Windows platform.
Fix some deprecation warnings on Windows

This fixes some snprintf and vsnprintf related deprecation
warnings we've been having on Windows with recent compilers.
Merge pull request #1373 from arrbee/why-cdecl-why

Why cdecl why?