prboom: fix static linking

Fixes:
http://autobuild.buildroot.net/results/97b/97bb50add010e44aca8fcb079d8df9270d3cae45/
http://autobuild.buildroot.net/results/a96/a969fd15a652df96c2414cf0c5f2e345d38e6e6c/
http://autobuild.buildroot.net/results/9ba/9ba92c01b2c6b6eae2dcda60c2b80d66d2002d4b/
http://autobuild.buildroot.net/results/776/776394ba89070ee2fa6e81585078df0a0e2ae3f2/



And many more.

SDL_mixer uses symbols from SDL, but ends up after it on the link cmdline,
breaking static linking. Fix it by forcing the SDL libs at the very end.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>