Work even if TRUE is undefined

Compiling on Ubuntu 12.10 with flags:

```
CC=gcc-4.5 CXX=g++-4.5 ./configure --enable-sdl-video --enable-sdl-audio --without-gtk --without-esd --enable-jit
```

fails with the error:

```
main_unix.cpp: In function ‘bool init_sdl()’:
main_unix.cpp:684:45: error: ‘TRUE’ was not declared in this scope
```

This happens because TRUE is undefined in these lines of code:

```
#ifdef USE_SDL_VIDEO
	// Don't let SDL block the screensaver
	setenv("SDL_VIDEO_ALLOW_SCREENSAVER", "1", TRUE);

	// Make SDL pass through command-clicks and option-clicks unaltered
	setenv("SDL_HAS3BUTTONMOUSE", "1", TRUE);
#endif
```

Switching to use ```true``` instead of ```TRUE``` works fine.