Expand CI coverage and move actions into scripts

This change makes a couple changes to the CI workflow:
 - Adds more compiler coverage:
     - gcc to MacOS (see note below)
     - 32 and 64bit gcc and clang to Windows

 - With more builds, this separates them into per-OS workflow YAMLs
   (laying the foundation for more build environments: BSD? DOS? ... )

 - Moves all functional commands from GitHub-syntax-YAML into scripts,
   which (besides eliminating repeated code), now serve a dual-purpose
   of being runnable outside of GitHub.
     - One script takes care of listing dependent packages for the given
       runtime environment
     - Another script takes care of configuring and building

These scripts can be leveraged by a nightly build & asset generator in
the future.

Note: adding GCC to MacOS is now "correct" from a build perspective,
however to keep this PR focussed on the CI workflow I have not included
the coreMIDI / AppleBlocks code-fixes here (so for now, the gcc macOS
builds will fail; we will merge the coreMIDI / AppleBlocks later
depending on how upstream wants to handle it).

scripts/count-warnings.py

@@ -23,7 +23,7 @@ import sys
 # then script will return with status 1. Simply change this line if you
 # want to set a different limit.
 #
-MAX_ISSUES = 375
+MAX_ISSUES = 420
 
 # For recognizing warnings in GCC format in stderr:
 #