setup.py fixes, guix and pyopencl
setup.py fixes, guix and pyopencl

- detect GUIX
- FreeBSD uses "pkg install", not "pkg_install"
- add pyopencl dependency checking

hb9kns/PyBitmessage,bmng-dev/PyBitmessage,hb9kns/PyBitmessage,hb9kns/PyBitmessage,bmng-dev/PyBitmessage,bmng-dev/PyBitmessage,hb9kns/PyBitmessage