Use new dune based build system. This no longer has a way to
enable the non-minimal build. As it turns out, though, no
package required that (and it was impossible to get a non-minimal
extlib build from OPAM before anyways).
The old expression needs to be retained for extlib 1.7.7 and just
moved over.
Unfortunately there's no way to get Haxe 4.0 and 4.1 to work with extlib
1.7.8 (not even without the minimal install), so we need to package
1.7.7 again, at least until 1.7.9 (?) brings backwards compatibility
packages, hopefully.
