Cleaned up OpenGL.h header.

I didn't know that C preprocessor had `#elif` until now. Learning
something new every day.

src/OpenGL.h

@@ -30,12 +30,22 @@
 
 #include "magnumConfigure.h"
 
+/* Desktop OpenGL */
 #ifndef MAGNUM_TARGET_GLES
 #include <GL/glew.h>
 #include <OpenGL/GL/glcorearb.h>
-#else
 
-#ifndef MAGNUM_TARGET_NACL
+/* NaCl has its own gl2.h, the official one causes linker issues. Additionaly
+   to NaCl's gl2ext.h we are including our own to prevent undeclared symbol
+   errors with some recent extensions. */
+#elif defined(MAGNUM_TARGET_NACL)
+#include <GLES2/gl2.h>
+#include <GLES2/gl2ext.h>
+#undef __gl2ext_h_
+#include <OpenGL/GLES2/gl2ext.h>
+
+/* Generic OpenGL ES */
+#else
 #include <OpenGL/KHR/khrplatform.h>
 #ifndef MAGNUM_TARGET_GLES2
 #include <OpenGL/GLES3/gl3platform.h>
@@ -45,16 +55,6 @@
 #include <OpenGL/GLES2/gl2.h>
 #include <OpenGL/GLES2/gl2ext.h>
 #endif
-
-/* NaCl has its own gl2.h, the official one causes linker issues. Additionaly
-   to NaCl's gl2ext.h we are including our own to prevent undeclared symbol
-   errors with some recent extensions. */
-#else
-#include <GLES2/gl2.h>
-#include <GLES2/gl2ext.h>
-#undef __gl2ext_h_
-#include <OpenGL/GLES2/gl2ext.h>
 #endif
 
 #endif
-#endif