Update testing matrices and fix warnings

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.

Refs #2899, #3006
Fixes #3007

Change-Id: I3a31a40c472d420adbb38d11ed5121702d7b3f7d