SDL2 support (mostly)

Makefile is adjusted, it compiles and works mostly, but
* For some reason (bug in SDL_GetRelativeMouseState() ?)
  mouse input doesn't work properly.. it seems to be bound
  to window borders, even if input is grabbed
* some keys can't be used anymore because there's no SDLK_*
  for them anymore (gotta find out if this is important)
* Maybe some of the changes need cleanup
Fixes for SDL2

* Fix input issues (mouse-wheel and mouse input)
* SDL2 is not default anymore in the Makefile (use WITH_SDL2=yes)
* If SDL2 is enabled, CD audio is disabled (SDL2 doesn't support
  that - use OGG/Vorbis instead)
* Small fix to make it compile with SDL1.2 again
SDL2: Fix Icon
Set gamma with SDL2
Fix printing of the "CDA is disabled" warning
use xrandr for x11gamma (still hacky)
Clean up + improve setting gamma via xRandR

* save gamma on start and restore on exit
* handle errors
* Restore SDL1.2 compatibility
Add -DSDL2 for Win32/OSX support (untested), print SDL2 usage on startup

The quake2 binary now gets -DSDL2 in the CFLAGS, so Win32/OSX can
use different #include paths accordingly.
This is also (ab)used to print which SDL version is used on startup.
Don't use this for anything else, use
#if SDL_VERSION_ATLEAST(2, 0, 0)
instead.

I haven't tested building on/for Win32 or OSX, there may be more
work to do.

Furthermore I added Copyright-Info about CalculateGammaRamp()
in refresh.c (it's from SDL2)
Rename CreateWindow() to CreateSDLWindow()

CreateWindow() is a symbol internal to Windows. We can't use it in out
code or it will collide.
Correct SDL2 statement for Windows

SDL2 can and should be statically linked. This is much more easier than
linking it dynamically and more comfortable for the user. One libs less
to care about.
Enable SDL2 by default
Use "directsound" as sounddriver when using SDL2 on Windows
Clarify SDL stuff in the README, mention vsync bug with SDL 1.2
Mention that OS X is unsupported

I neither have a Mac nor the time to maintain this platform. SDL 2.0
requires some more complex changes than the usual "two lines to track
Unix and Windows" so suspend OS X support until someone steps up.
Mention SDL 2.0 in the Makefile header
Add a FAQ regarding XInput 2.2 or higher needed by SDL 2.0
Merge branch 'sdl2'
Add $ORIGIN/lib the RPATH on FreeBSD and Linux

This should ease the overwriting of systemwide installed libs with
specialized versions. Also distributors may use this facility to supply
libraries next to the binaries. Maybe the same should be added for the
other platforms, but I'm not sure if and how they support RPATHes.