Merging in latest from upstream (HDFFV/hdf5:refs/heads/develop)
* commit '22d78bdfa4d560a0b8a2dbb547574bfe61bf64f0':
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.
