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/nsgl_context.m

@@ -28,6 +28,8 @@
 
 #include "internal.h"
 
+#if defined(_GLFW_COCOA)
+
 #include <unistd.h>
 #include <math.h>
 
@@ -374,3 +376,5 @@ GLFWAPI id glfwGetNSGLContext(GLFWwindow* handle)
     return window->context.nsgl.object;
 }
 
+#endif // _GLFW_COCOA
+