Update to latest autotools (autoconf-2.69, automake-1.15)

[sjnewbury]

Hi AndyA,

I don't know if you're still working on maintaining this fork, but I've updated to work with modern autotools. I'll probably port it to SDL2 if I find the time/inclination, and/or there's interest in having it happen.

I'm also preparing a Gentoo ebuild which I'll carry in one of my overlays.