Fix launching ioq3 on Linux

Q_OS_X11 does not exist (it was a Qt documentation error).
Q_OS_UNIX is also defined on Mac, so move Unix block after Mac.
There is a specific Q_OS_LINUX, but it's a general Unix style command.

Also added error message for unhandled platforms.
Merge pull request #4 from zturtleman/fix_linux

Fix launching ioq3 on Linux