The old system of just defining macros available in some other platform
was susceptible to silently using the wrong code if you forgot to
include a particular header. This fixes a crash on non-Windows platforms
introduced by e1fbac3c.

Symbian, Xbox, Blackberry and iOS got removed.
FreeBSD and Android kept due to them potentially being able to run Citra in the future.
The iOS specific part also got removed from PPSSPP in order to fix a bug there.

Also replaced C headers with the C++ equivalent ones