Get hyper_function.h compiling on GCC 4.6.

We must avoid the following features:
- the `using` syntax for typedefs
- alias templates (so, rename `hyper_function` to `function`)
- the `override` keyword
- defaulted virtual destructors

sysconfig.h

@@ -72,10 +72,6 @@
 #define NOLICENSE ISSTEAM
 #endif
 
-#ifndef USE_STDFUNCTION
-#define USE_STDFUNCTION ISSTEAM
-#endif
-
 #ifndef CAP_SHADER
 #define CAP_SHADER CAP_GL
 #endif