add cmake support

[illwieckz]

• import cmake support by @boxerab from https://github.com/GrokTexture/crunch/tree/unity-custom
• do not build Windows-centric thread stuff outside of Windows
• look for cmake's Threads package to properly link to pthread on Linux

extra:

• add clean-up target to legacy Makefile

See also that alternative (some bits can be interesting) on the old binomial branch: madebr@eea5a8d

[DolceTriade]

Is this actually required?

[illwieckz]

I don't know, it's a line from that foreign branch I merged

[boxerab]

This forces C++14 standard. You probably don't want that :)

[illwieckz]

@boxerab Ok thank you for the prompt answer, I'll remove it then :-)