[depends] add (non-native) sdl to linux
[depends] steal X11 and friends from system libs on linux
they are needed, but not in unified deps
[depends] fix ffmpeg cross compilation on linux
[buildsteps] fix linux64 build
[buildsteps] if multithreaded building fails on linux, rerun make singlethreaded once more
[buildsteps] there is no package on linux64 build
Merge pull request #3250 from wsnipex/buildsteps

fix linux building with unified deps