move configurables into variables
macosx: set minimum to leopard
macosx: separate liblzma package
Windows: Add Windows support to tuklib_cpucores().

It is used for Cygwin too. I'm not sure if that is
a good or bad idea.

Thanks to Vincent Torri.
Fix the previous commit which broke the build.

Apparently I didn't even compile-test the previous commit.

Thanks to Christian Hesse.