opam 1.1.2

This switches the build system to a more conventional `make`-based
one.  The new rules also unbreak the `--HEAD` installation target
for the forthcoming 1.2.0 release.

Closes #31818.

Signed-off-by: Anil Madhavapeddy <anil@recoil.org>
Signed-off-by: Jack Nagel <jacknagel@gmail.com>