[CMake] Update dependency finders.

1. Fixes CMake using any found HarfBuzz version lower than the minimum
required. This is based on HALX99's merge request at
2. Update FindHarfBuzz.cmake from
and guard post-CMake-3.1 features to keep the minimum version unchanged
3. Update FindBrotliDec.cmake to stop the warnings, based on what
is doing

* CMakeLists.txt, builds/cmake/FindHarfBuzz.cmake: Implement 1 and 2.
* builds/cmake/FindBrotliDec.cmake: Implement 3.
4 files changed