Refactor and expand the CI workflows

- For each OS, builds of the default compiler plus the
  latest-supported compilers are run. When multiple operating systems
  are supported (such as Ubuntu 16.04 and latest), a build on
  the oldest OS using its default compiler is also performed.

- Debug builds are used because they often are more thorough at
  detecting coding issues (debug warning counts are higher).

- Runtime dynamic sanitizers are added and serialized per-compiler.
  Their build and runtime log-files are xz-compressed, and then
  GitHub's asset upload Zips the log directory.

- Each workflow now holds the maximum allowed compiler warnings
  per-build, so we can have tighter control of when new warnings
  are introduced (that would otherwise pass if still below the
  maximum)

- Use of github's new 'cache' feature has been leveraged to restore
  the brew, macports, and msys2 environments to eliminate the
  lenghthy setup times for those environments.  If a new cache
  is needed, then we simply increment the cache `key:` value and
  the next CI run will archive new caches. (Note that GitHub has a
  400MB limit on cache size however they have already said they
  are raising it - so we might be able to cache out longest running
  job which is MSYS+Clang)

- Where it makes sense, multi-line workflow statements have been
  broken out into .github/scripts as files to make the workflow YAML
  leaner and more readable, while giving us a richer environment in
  the scripts.

.github/scripts/shrink-msys2.sh

@@ -0,0 +1,44 @@
+#!/bin/bash
+
+# Copyright (c) 2019 Kevin R Croft <krcroft@gmail.com>
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+# This script reduces the size of an MSYS2 and MingGW installation
+# under Windows. Its main use is to shrink these areas to fit within
+# GitHub's cache limits; however it can also be used by end-users wanting
+# to save space.
+#
+# Usage: ./shrink-msys2.sh
+#
+set -xuo pipefail
+set +e
+
+# Clean all the package archives from pacman
+pacman -Scc --noconfirm
+
+# Strip binaries using their associated striping tool
+for dir in /usr /mingw32 /mingw64; do
+
+	# Enter our directory if we have it
+	cd "${dir}" || continue
+
+	# Check if we have an associated stripping tool
+	if [[ ! -f "bin/strip.exe" ]]; then continue; fi
+
+	# Start stripping
+	find . -type f              \
+		\( -iname '*.exe'       \
+		   -or -iname '*.a'     \
+		   -or -iname '*.dll'   \
+		   -or -iname '*.so'    \
+		\)                      \
+		-a ! -iname 'strip.exe' \
+		-print0                 \
+	| xargs -0 ./bin/strip.exe --strip-unneeded
+done
+
+# Delete MinGW's share directories
+rm -rf /mingw*/share
+
+# This entire script is best-effort, so always return success
+exit 0