cmake: fix build on archlinux

 - add "ARCHLINUX" cmake option
 - add `-lpthread` to scsynth and sclang link flags
 - fix "undefined reference to symbol 'pthread_mutexattr_settype@@GLIBC_2.2.5'" error

Reported in [this thread](http://new-supercollider-mailing-lists-forums-use-these.2681727.n2.nabble.com/link-failure-for-pthreads-on-Arch-Linux-td7594503.html)