[waffle] Use cmake config files instead of find

[Dylan Baker]

This is the 3rd, and hopefully final, iteration of these patches.

Changes from version 2:
- rearrange the new code to be with the pkgconfig code in the cmake
  file
- use CMAKE_BINARY_DIR
- Drop patch 3 (use windows registry)

I have tested this on linux with ninja and make backends with both in
tree and out of tree builds.

This is available at my github:
https://github.com/dcbaker/waffle submit/cmake-config