Alire supports various linuxes, windows and macOS (I'm not sure about
arch linux, work is definitely in progress).

I put myself in as maintainer in alire.toml; they do require an email
address for the 'maintainers' table, but since it's quite unlikely (?)
we'll want to release an Alire crate for this example it may not
matter.

You may want to move main.adb and the executable back to ada/.

  * ada/.gitignore: ignore alire-related directories.
  * ada/README.md: updated for Alire.
  * ada/alire.toml: new.
  * ada/main.gpr: added Alire-related options.
  * ada/src/main.adb: renamed from ada/main.adb. Tidied up compiler
      warnings generated in Alire's 'development' profile.

My first rework changed the name of the main program from Main to
Ada_Example. On second thoughts, I reverted to Main, but forgot to
clean the directory, so that config/ada_example_config.gpr was still
present during tests.

  * ada/main.gpr: config/ada_example_config.gpr changed to
      config/main_config.gpr.
      In package Compiler, the Ada_Compiler_Switches now come from
      Main_Config.

Fix misnamed Ada project file.

Uses alire to do the builds, in job 'alire'.

Includes a temporary job, 'alire-macos-13', which builds ada/ using
macOS 13 and the latest stable Xcode. This is to check for possible
effects of the Xcode 15 change to 'ld', which has been found to
interfere with Ada exception handling. There isn't any exception
handling in this code, so there _should_ be no impact.

  * .github/workflows/main.yml
    (alire): new. Builds ada/ for windows-latest, ubuntu-latest,
      macos-latest.
    (alire-macos-13): new. Builds ada/ for macos-13/xcode-latest.

Add CI for ada/.