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_init.m

@@ -27,6 +27,9 @@
 //========================================================================
 
 #include "internal.h"
+
+#if defined(_GLFW_COCOA)
+
 #include <sys/param.h> // For MAXPATHLEN
 
 // Needed for _NSGetProgname
@@ -690,3 +693,5 @@ void _glfwTerminateCocoa(void)
     } // autoreleasepool
 }
 
+#endif // _GLFW_COCOA
+