use ocamlfind instead of opam to discover where ocaml-src is located

[hannesm]

and where to install and uninstall to

Discussed in #134 (comment) and #134 (comment) by @shym and @dinosaure

[shym]

I wonder if using $(ocamlfind query ocaml-src) to find parent directories is not a bit fragile: if this is not in an OPAM-like setup, we could end up installing and removing stuff in wrong directories. I would be more comfortable for those scripts to error out if the prefix is not explicitly set.

[dinosaure]

May be ocamlfind printconf destdir is preferable. However, prefix seems highly related to the OPAM layout.

[dinosaure]

We should probably also just fail if $PREFIX is not set.

[hannesm]

thanks for your feedback, I removed uninstall (it is unused anyways), and adapted install.sh to use $PREFIX.

[hannesm]

the remaining question I have is how defensive should install.sh be? should we warn and fail on all potential issues (PREFIX being undefined, MAKE being undefined, ...)? Or is it fine to just use the empty prefix, an undefined MAKE, ...? Since it is called by make install (and there's no expectation to call it differently), I'm fine with the no error handling approach as proposed in this PR.

If there's some strong opinion to act differently, please raise your voice - best with a brief explanation why it should be more defensive.

[shym]

This looks good to me. I agree with the approach: install.sh doesn’t need to accomodate for direct invocation and so $PREFIX should be empty only when the user has explicitly set it so in configure.sh, so there should be no surprise there.

[hannesm]

Thanks, squash-merged.