Add SDL to the travis script.

[SiegeLord]

No description provided.

[SiegeLord]

Current issues:

• No dialog addon, so can't do the install test on Linux
• Doesn't build on OSX

[dos1]

I've looked into SDL on macOS.

• the first issue is that the build system sets ALLEGRO_MACOSX and appends macOS files to the build
• after fixing that, build fails on including GL headers, because the correct ones are behind ALLEGRO_MACOSX ifdef...
• after fixing that too, it conflicts with the dialog addon
• after disabling native dialogs, it builds :P

[SiegeLord]

Ah, going above and beyond I see. Don't worry about it too much, I just wanted to note down what issues I hit so I remembered what was up if I returned to it. Thanks for investigating things regardless!

[dos1]

Well, it just made me curious. I have a pretty convenient cross-compilation environment already setup, wouldn't bother otherwise :D