Deal with system compilers lacking compiler-libs

[kit-ty-kate]

Currently the ocaml-system package expects compiler-libs to be installed if ocaml is installed.
However on some platforms this is not the case.

It would be nice to have at least a detection of the issue early (e.g. a meta base-compiler-libs or something that checks that the package is here), otherwise packages will fail with weird error messages for people who don't know the OCaml compiler internals, such as:

#=== ERROR while compiling ocamlfind.1.9.6 ====================================#
# context     2.2.1 | linux/x86_64 | ocaml-system.5.1.1 | https://opam.ocaml.org#accd28112a6751e427167bf0056618f09868d85f
# path        ~/.opam/5.1.1/.opam-switch/build/ocamlfind.1.9.6
# command     ~/.opam/opam-init/hooks/sandbox.sh build make all
# exit-code   2
# env-file    ~/.opam/log/ocamlfind-131493-b2399b.env
# output-file ~/.opam/log/ocamlfind-131493-b2399b.out
### output ###
# [...]
# Warning 6 [labels-omitted]: label preds was omitted in the application of this function.
# ocamlc -I +compiler-libs  -o ocamlfind -g findlib.cma unix.cma \
#            -I +unix -I +dynlink ocaml_args.cmo frontend.cmo
# ocamlc -I +compiler-libs -opaque   -I +unix -I +dynlink -c topfind.mli
# ocamlc -I +compiler-libs -opaque   -I +unix -I +dynlink -g -c topfind.ml
# File "topfind.ml", line 49, characters 4-25:
# 49 |     Topdirs.dir_directory d;
#          ^^^^^^^^^^^^^^^^^^^^^
# Error: Unbound module Topdirs
# make[1]: *** [Makefile:176: topfind.cmo] Error 2
# make[1]: Leaving directory '/home/aggressive_racer1/.opam/5.1.1/.opam-switch/build/ocamlfind.1.9.6/src/findlib'