Makefile.darwin: target 10.5 when building for x86 against SDL2.
build_cross_osx-sdl2.sh: new script for me to cross-build on my linux.


git-svn-id: svn://svn.code.sf.net/p/quakespasm/code/trunk@1034 af15c1b1-3010-417e-b628-4374ebc0bcbd