WIN32_THREADS -> WIN32

Fixes nasty bug where some parts of the code didn't define WIN32_THREADS
and were using the integer mutex implementation, causing even confusion
to the debuggers.

And there is little interest of other thread implemenation on Win32
besides Win32 threads.

src/mapi/glapi/SConscript

@@ -16,7 +16,6 @@ if env['platform'] != 'winddk':
         env.Append(CPPDEFINES = [
             '_GDI32_', # prevent gl* being declared __declspec(dllimport) in MS headers
             'BUILD_GL32', # declare gl* as __declspec(dllexport) in Mesa headers
-            'WIN32_THREADS', # use Win32 thread API
         ])
 
     env.Append(CPPPATH = [