Folding support to undefined code

Hi,
it would be nice if you could hide code that is undefined ("flagged code", i.e. flag set for compiler).

This could be so that you could hide undefined code same way as you can hide comment lines, so that we have this [+] and [-] in the margin.

So it could be useful to hide for example this kind of code:

#if (SOMETHING == ON)
void funtion1(int);
void funtion2(int);
#endif