Add giftware license from Allegro 4.

This seems correct since a lot of the code in this project comes
straight from Allegro 4.
Detect and link with Allegro 5 libraries.
Do not detect and link with Allegro 4.
Add definitions of stricmp/strlwr/strupr from A4.
Build with allegro4-on-5.

Code which doesn't compile is simply hacked out with ifdefs.
Delete deliberate 1.5s delay at startup.
Figure out how to replicate the lighting effect with A5.
Replicate shaky spotlight effect when firing.
Replicate spotlight effect on menu.
Ensure colour 0 is opaque black in backgrounds.
Fix mouse cursor so that transparent pixels remain so.
Use CONVERT_8BIT_INTENSITY to convert light maps.
Force help bitmaps to be converted.
Implement fade-ins which can't be done in the allegro4-to-5 wrapper.
Remove 'scanlines' option.

It wasn't supported on most platforms anyway.  It would be possible and
easy to implement with shaders now, but it's more of a gimmick than a
useful effect.
Remove 'filtered blit' support.
Remove 'family' option.
Play intro music using allegro_acodec.
Remove random mod music playback support.
Delete JGMOD code.