wgl: Enable the use of Win32 threads.
diff --git a/src/mesa/SConscript b/src/mesa/SConscript
index a878d31..5cf3bfd 100644
--- a/src/mesa/SConscript
+++ b/src/mesa/SConscript
@@ -16,6 +16,7 @@
 		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
 		])	 
 
 	#