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/build.md

@@ -0,0 +1,110 @@
+# DOSBox build script
+
+A helper-script that builds DOSBox with varying compilers, release types, and versions
+on MacOS, Linux, and Windows.
+
+A second script, **list-build-dependencies.sh** prints a list of package and library
+dependencies used to build and link DOSBox, customized for your hardware, operating system,
+and selected compiler and its version.  You can use this script's output to install
+those necessary packages.
+
+## Requirements
+
+- **Windows newer than XP**
+    - **NTFS-based C:**, because msys2 doesn't work on FAT filesystems
+	- **Chocolately**, to install msys2
+
+- **MacOS** 10.x
+	- **brew**, as the package manager
+
+- **Ubuntu** 16.04 or newer
+	- **apt**, as the package manager
+	- **sudo**, to permit installation of depedencies (optional) 
+
+- **Build dependencies (all operating systems)**
+    - Per those listed by the accompanying list-build-dependencies.sh script
+
+## Windows Installation and Usage
+
+1. Download and install Chocolatey: https://chocolatey.org/install
+1. Open a console and run Cholocatey's command line interface (CLI) to install msys2:  
+   `choco install msys2 git --no-progress`
+
+    ```
+    Chocolatey v0.10.15
+    Installing the following packages:
+    msys2 git
+    By installing you accept licenses for the packages.
+    
+   msys2 v20180531.0.0 [Approved]
+   msys2 package files install completed. Performing other installation steps.
+   Installing to: C:\tools\msys64
+   Extracting 64-bit C:\ProgramData\chocolatey\lib\msys2\tools\msys2-base-x86_64-20180531.tar.xz to C:\tools\msys64...
+   C:\tools\msys64
+   Extracting C:\tools\msys64\msys2-base-x86_64-20180531.tar to C:\tools\msys64...
+   C:\tools\msys64
+   Starting initialization via msys2_shell.cmd
+   PATH environment variable does not have C:\tools\msys64 in it. Adding...
+   ```
+
+1. Open a new console and clone the DOSBox staging repository:  
+   `git clone https://github.com/dreamer/dosbox-staging.git`
+
+1. [optional] Install the build tools and package dependencies, if not yet done so:
+   ``` shell
+   cd dosbox-staging
+   SET CWD=%cd%
+   bash -lc "pacman -S --noconfirm $($CWD/scripts/list-build-dependencies.sh)"
+   ```
+
+1. Launch the build script with default settings:
+   ``` shell
+   cd dosbox-staging
+   SET CWD=%cd%
+   bash -lc "$CWD/scripts/build.sh --bin-path /mingw64/bin --src-path $CWD"
+   ```
+
+## MacOS Installation and Usage
+
+1. Download and install brew: https://brew.sh
+1. Install git: `brew install git`
+1. Clone the repository: `git clone https://github.com/dreamer/dosbox-staging.git`
+1. Change directories into the repo: `cd dosbox-staging`
+1. [optional] Install the build tools and package dependencies, if not yet done so:
+   ``` shell
+   brew update
+   brew install $(./scripts/list-build-dependencies.sh)
+   ```
+1. Build DOSBox: `./scripts/build.sh`
+
+## Linux (Ubuntu/Debian-based) Installation and Usage
+
+1. Install git: `sudo apt install -y git`
+1. Clone the repository: `git clone https://github.com/dreamer/dosbox-staging.git`
+1. Change directories into the repo: `cd dosbox-staging`
+1. [optional] Install the build tools and package dependencies, if not yet done so:
+   ``` shell
+   sudo apt update -y
+   sudo apt install -y $(./scripts/list-build-dependencies.sh)
+   ```
+1. Build DOSBox: `./scripts/build.sh`
+
+## Additional Tips
+
+The compiler, version, and bit-depth can be selected by passing the following common
+options to the **list-build-dependencies.sh** and **build.sh** scripts:
+* `--compiler clang`, to use CLang instead of GCC
+* `--compiler-version 8`, to use a specific version of compiler (if available in your package manager)
+* `--bit-depth 32`, to build a 32-bit binary instead of 64-bit
+
+After building, your binary will reside inside the src/ directory.
+
+Build flags you might be interested in:
+* `--release debug`, to build a binary containing debug symbols (instead of **fast** or **small**)
+* `--lto`, perform optimizations across the entire object space instead of per-file (Only available on Mac and Linux)
+
+The above flags are othogonal and thus can be mixed-and-matched as desired.
+
+If you want to run multiple back-to-back builds from the same directory with different settings then
+add the `--clean` flag to ensure previous objects and binaries are removed.
+