Task #2899: Update testing matrix versions for GROMACS 2020 release
Install and update testing to add gcc-9
To make sure we continue testing the latest version of gcc, we need to have gcc-9 installed on at least one build machine.
Update testing matrices and fix warnings
Updated to test gcc 9, clang 8, and cmake 3.15.1.
Also fixes for compilation with gcc 9 and clang 9 (still in release
candidate, not yet released)
Added a gmx_source_file_warning_suppression() CMake helper function
(like the recently introduced gmx_target_warning_suppression()) to
manage per-source-file suppressions. This is used to suppress a
warning about the abuse of t_inputrec that is too difficult to fix
without replacing all of the handling of initialization and broadcast
of t_inputrec and subfields.