cleanup USE_GAMMA_RAMPS and USE_SDL2 stuff

git-svn-id: svn://svn.code.sf.net/p/quakespasm/code/trunk@1051 af15c1b1-3010-417e-b628-4374ebc0bcbd
exec default.cfg and config.cfg upon in-game gamedir changes.

git-svn-id: svn://svn.code.sf.net/p/quakespasm/code/trunk@1052 af15c1b1-3010-417e-b628-4374ebc0bcbd
add code for USE_GAMMA_RAMPS case for SDL2 builds. (still off by default.)

git-svn-id: svn://svn.code.sf.net/p/quakespasm/code/trunk@1053 af15c1b1-3010-417e-b628-4374ebc0bcbd
rename gnu.txt to LICENSE.txt

git-svn-id: svn://svn.code.sf.net/p/quakespasm/code/trunk@1054 af15c1b1-3010-417e-b628-4374ebc0bcbd