Fix for autoconf with Allegro backend

[ceeac]

When using autoconf for generating config.default on Ubuntu, and Allegro is installed but not SDL(2), configure falls back to the headless server build even though there is a working graphics library installed.
This is because on Ubuntu, the Allegro library is named liballeg.so which autoconf does not pick up.
The attached patch aims to fix this behaviour by searching for both liballegro.so and liballeg.so.

[Ters]

Huh? liballeg is what I remember it being called twenty years ago. This was on DOS, so it couldn't be any longer. It looks like the name might have changed in Allegro 5. Which version(s) of Allegro do(es) Simutrans work with? Which version does Ubuntu come with?

[ceeac]

Simutrans only works with Allegro 4; on Ubuntu, you can install both version 4 and 5, however the Allegro folks advise against installing both at the same time.

[prissi]

Does the allegro backend still work? The last time I tried it, I could not even link it properly (on windows though).

[ceeac]

For me it does work, however there are some bugs (for example, the window cannot be resized). I normally do not use the Allegro backend either, so maybe it could also be removed if no-one uses it?

[Ters]

I think it is primarily for Haiku.

[Isaac Eiland-Hall]

Rare the code for poems
But Haiku is important
We keep Allegro