Merge pull request #1788 in HDFFV/hdf5 from ~KOZIOL/hdf5:feature/update_gcc_flags to develop
* commit '73df02f13c2b1e0b256cdbff1c7c584e84548df4':
Updates to warnhist script, along with a few cleanups, and add some comments to warning cleanups that are a bit obscure.
Cleaned up code that genericizes warning output, and add "-u" option to leave unique types in warnings (i.e. don't genericize output). Also fixed the '-S' option to work correctly.
Added GCC9 flags to CMake build
Add support for GCC9, update warnhist script, and clean up warnings.
