Fix building for Android with standard build systems.

In this case, standard doesn't mean that we can expect sane behavior.

src/Magnum/Extensions.h

@@ -33,6 +33,14 @@
 
 namespace Magnum {
 
+/* Standard Android build system thinks that it's okay to define unmangled
+   unprefixed macros. I think that whoever did that needs to be punished,
+   becuase I am then not able to use that identifier for extension names.
+   Use CORRADE_TARGET_ANDROID here instead. */
+#ifdef ANDROID
+#undef ANDROID
+#endif
+
 /**
 @brief Compile-time information about OpenGL extensions