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_monitor.m

@@ -29,6 +29,8 @@
 
 #include "internal.h"
 
+#if defined(_GLFW_COCOA)
+
 #include <stdlib.h>
 #include <limits.h>
 #include <math.h>
@@ -625,3 +627,5 @@ GLFWAPI CGDirectDisplayID glfwGetCocoaMonitor(GLFWmonitor* handle)
     return monitor->ns.displayID;
 }
 
+#endif // _GLFW_COCOA
+