[AD] CMake Find* macros for Allegro

[Evert Glebbeek <eglebbk@xxxxxxxxxx>]

It would be nice to have a collection of CMake Find* macros for use projects that use Allegro. Attached is the set I use in my projects, and I thought they might serve as a basis for a more complete and elaborate set.

Currently, the main library as well as each addon has its own macro. In addition to this, it may be desirable to have one that links to Allegro as well as all addons (including Main if needed). When I started this I didn't feel like doing that and I only needed Allegro plus one or two addons. Now, I need a whole bunch more, so I end up with a lot of nearly-identical ugly boiler-plate code in my main CMakeLists.txt.

Last but not least, this uses the CMake pkg-config package to set things up. That means it only works on platforms where we use pkg-config, in other words, OS X and Linux. For Windows specifically we'd need to do something else. Since I don't have Windows, I don't know where or how to set that up, so any help there would be appreciated by me independently of whether we decide to include something like this with Allegro.

This only addresses CMake, of course. We may wish to provide M4 macros or similar for other build systems, but the problem is someone needs to make them.

Evert