Add a minimal CMake build that gets the cassowary library building, although without any config values set
Turn on GTL build - since GTL isn't developed any more, probably not much point in looking for a system installed version.
Move headers to include dir, remove old build system files
Tell src dir about the new include location
Add flex and bison flags
Separate out the tests, move some other files around
Rework headers not to explicitly call out include subdirectory
Link gtl to cassowary