cmake infrastructure

[flriancu]

Related to #19 . Since I was already finished with the PR, I thought I might as well file it.

I've split the commits so you can cherry-pick individual changes as you see fit. Changes are listed in the commit messages.

Tested on Windows (cl v14.00 via vs2015, cl v14.28 via vs2019, clang-cl, clang, mingw-gcc), Linux (clang, gcc), macOS (clang).