Support multi-versioned installation

"Multiver" is a convention for installing more than one version of a package on the system. If this scheme is enabled (and it will be an option, whose default is off), then the header-files are installed to `${prefix}/include/opm-2013.09` and the libraries to `${prefix}/lib${whatever}/opm-2013.09`. This option will be used when building backport packages.