Update CMake dependency finders
- Update FindHarfBuzz.cmake to what WebKit uses and guard post-CMake-3.1 features to keep the minimum version unchanegd
- Update FindBrotliDec.cmake to stop the warnings, based on what https://github.com/google/woff2/blob/master/cmake/FindBrotliDec.cmake is doing
- Include !31 (closed) to elegantly skip over HarfBuzz if it is found, but the minimum required version is not met.
Closes !31 (closed).