Simplified build dependencies for linux

compile_msound_linux does not rely on PortAudio sources here.
Installing the library systemwide (using the distro's package
manager or manually) does suffice. Tested on up-to-date Arch
Linux, using PortAudio v19_20140130-1.
Merge pull request #6 from Janwillhaus/fix_linux_compile

Simplified build dependencies for linux