[feature] compiler option: --minimal-version for Makefile
The purpose is to get quickly a clear error message about the version
of the compiler, rather than a strange error message caused by
an update in the compiler and/or in the stdlib.
It is recommended to use this option in your Makefile for applications
using new features, or new functions of the stdlib, or in any application
published somewhere, and potentially compiled by people having
wild and heterogeneous versions of Opa (nightly builds, from sources).