add sdl-1.2.15 as default 1.2 build.

This build is more recent than 1.2.14 and fixes many of the previous
bugs that we had to manually patch in 1.2.14.
add mercurial/sdl-1.2 as the default 1.2 build

This version contains even more bug fixes that are required to build at
least on OSX.  Hopefully this is true for all other systems as this
should really just be a bug fix branch.
add flag to rebuild libs only when requested.

Previously we would always rebuild everything every time that build.sh
was called which could be a bit annoying if you have a failing build.
Now it will track which ones have successfully built and only rum them
against if requested.