Small fixes to support the Debian/GNU Linux distro. Hopefully Ubuntu too.

Hi!

I've added a new makefile target (`PLATFORM_LINUX_DESKTOP`; I hope you're OK with the name) to allow a clean building under Debian. I suppose that should cover all debian-derived distros out there. I've also included `alloca.h` for the non-ANSI `alloca` calls in `stb_vorbis.c` conditional to compilation under linux.

Cheers!