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/cocoa_window.m

@@ -28,6 +28,8 @@
 
 #include "internal.h"
 
+#if defined(_GLFW_COCOA)
+
 #include <float.h>
 #include <string.h>
 
@@ -2047,3 +2049,5 @@ GLFWAPI id glfwGetCocoaWindow(GLFWwindow* handle)
     return window->ns.object;
 }
 
+#endif // _GLFW_COCOA
+