[opam 2.1~alpha4] ./configure silently disables mccs if a C++ compiler is not present

[kit-ty-kate]

$ which g++
$ git clone git://github.com/ocaml/opam.git
$ cd opam
$ ./configure && make lib-ext && make && sudo make install
$ opam --version
2.1.0~beta4
$ opam init
[NOTE] Will configure from built-in defaults.
Checking for available remotes: rsync and local, git.
  - you won't be able to use mercurial repositories unless you install the hg command on your system.
  - you won't be able to use darcs repositories unless you install the darcs command on your system.
[ERROR] No external solver found. You should get one of aspcud, packup and mccs, or use a version of opam compiled
        with a built-in solver (see http://opam.ocaml.org/doc/External_solvers.html for details)
Continue initialisation anyway ? [Y/n]

[dra27]

Checks for 2.2.0:

• --with-mccs should cause an error in this case (that should be working?)
• make cold should force this - i.e. mandate that it builds opam with a solver

[rjbou]

fixed by #5527