Add conditional compilation for platform units

This is a step towards being able to compile GLFW manually without
needing to duplicate a lot of platform- or OS-specific logic.

src/win32_thread.c

@@ -29,6 +29,8 @@
 
 #include "internal.h"
 
+#if defined(GLFW_BUILD_WIN32_THREAD)
+
 #include <assert.h>
 
 
@@ -96,3 +98,5 @@ void _glfwPlatformUnlockMutex(_GLFWmutex* mutex)
     LeaveCriticalSection(&mutex->win32.section);
 }
 
+#endif // GLFW_BUILD_WIN32_THREAD
+