Use SDL_main on all platforms.
Fix main() for non-const argv so it matches with the SDL
prototype.
Adapt win32 WinMain() to main() and get rid of the win32 special
case in Common::Init().
Don't include the lazy precompiled.h everywhere, only what's
required for the compilation unit.
platform.h needs to be included instead to provide all essential
defines and types.
All includes use the relative path to the neo or the game
specific root.
Move all idlib related includes from idlib/Lib.h to precompiled.h.
precompiled.h still exists for the MFC stuff in tools/.
Add some missing header guards.
