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/wgl_context.c

@@ -29,6 +29,8 @@
 
 #include "internal.h"
 
+#if defined(_GLFW_WIN32)
+
 #include <stdlib.h>
 #include <assert.h>
 
@@ -776,3 +778,5 @@ GLFWAPI HGLRC glfwGetWGLContext(GLFWwindow* handle)
     return window->context.wgl.handle;
 }
 
+#endif // _GLFW_WIN32
+