improve camlp4+system installation experience

[edwintorok]

1. On CentOS/Fedora please suggest to install ocaml-camlp4-devel

   This would be the equivalent of camlp4-extra in Debian.
   The current message is:

   # * Debian/Ubuntu: sudo apt-get install camlp4-extra
   # * RHEL/CentOS/Fedora: sudo yum install ocaml-camlp4
   

2. Please suggest reinstalling ocamlfind, or fix ocamlfind automatically

   See the report on the mailing list and the solution.
   It'd be best if installing the camlp4+system package would do whatever is needed for ocamlfind to find the package:

   • could it install the required META files for camlp4?
   • should it suggest to the user to manually run opam reinstall ocamlfind? (probably not desirable as it involves recompiling almost everything)
   • should it trigger the rebuild of ocamlfind on its own? (not sure if its even possible, and see previous point)

3. Improve the check-camlp4.sh script

   To also check that ocamlfind can find the camlp4 package.

Testcase using docker (original report is about Arch, I use CentOS7 here for simplicity),
see how ocamlfind query camlp4 only works after ocaml-camlp4-devel system package is installed, camlp4 opam package is installed, and ocamlfind is reinstalled:

$ cat >Dockerfile <<EOF
FROM centos:latest
RUN cd /etc/yum.repos.d/ &&\
    curl -L -O http://download.opensuse.org/repositories/home:ocaml/CentOS_7/home:ocaml.repo &&\
    yum install -y ocaml opam m4 make patch gcc git
RUN opam init -y
RUN opam install ocamlfind
RUN yum install -y ocaml-camlp4
RUN opam install camlp4 || yum install -y ocaml-camlp4-devel
RUN opam config exec -- ocamlfind query camlp4 || opam reinstall ocamlfind
RUN opam config exec -- ocamlfind query camlp4
EOF
$ sudo docker rmi test-camlp4; sudo docker build -t test-camlp4 .

Output is available in this gist

@diml: thoughts?

[samoht]

Can you report that issue in the camlp4 bug tracker? Thanks!