dev-lang/ocaml: Bump to version 4.09.0, EAPI 7

dev-lang/ocaml: Bump to version 4.09.0, EAPI 7

This new OCaml ebuild uses EAPI 7. As discussed with Zlogenem, multi-
platform support has been removed and the ebuild is now keyworded for
~amd64 only.

Significant time was spent figuring out why the build failed when run
via ebuild, but succeeded when run manually. After I filed an upstream
issue[1] and a subsequent discussion, it turned out that Gentoo's ${P}
variable clashed with the OCaml build, as I had suspected. The line

    P ?= stdlib__

in a Makefile resulted in wrong file paths which in turn broke the
OCaml documentation build. I have asked upstream if their use of the
"?=" operator is deliberate; an answer is pending.

The license in this build has been changed to match [2].

[1] https://github.com/ocaml/ocaml/issues/9189
[2] https://ocaml.org/docs/license.html

Closes: https://bugs.gentoo.org/688108
Package-Manager: Portage-2.3.79, Repoman-2.3.16
Signed-off-by: Ralph Seichter <gentoo@seichter.de>
Signed-off-by: Mikle Kolyada <zlogene@gentoo.org>