Do not force CMAKE_INSTALL_PREFIX

It's /usr/local by default anyway, and setting disallows distro-specific
defaults or user override.
sys/sysinfo.h is specific ot Linux, not gcc
include cmath for sqrtf
Include local directories before everything else

This fixes build failures because of picking up system includes
instead of similarily named local ones
Enable warnings with clang as well
Merge pull request #307 from AMDmi3/freebsd-fixes

FreeBSD fixes
Create pl_PL
Delete pl_PL
English to Polish translation
Merge pull request #309 from kuzyn1/translation

Polish translation
Fix categies in .desktop file

- Move Game before Network as this is primarily game and shouldn't
  appear along with network utilities
- Add more precise ActionGame category