Support installation in a versioned directory

If the option USE_VERSIONED_DIR is set to ON (default is OFF), then
most files are installed in a ${suite}-${label}/ subdirectory
(e.g. header files go in "/usr/include/opm-2013.03/"). This enables
us to easily install backports of newer versions on systems which
have a system package for an older one, without getting conflicts.
Allow spec of not commonly rooted incl and lib dir

Header and library directory may be specified separately by using the
variables e.g. OPM_CORE_INCLUDE_DIR and OPM_CORE_LIB_DIR. These override
even the OPM_CORE_ROOT. This allows us to use a backported version which
uses versioned directory names.
Alert package maintainer about versioned dir option

If someone is going to backport these packages, they may have an
interest in using versioned directories. These will most likely
attempt to set directories on the build command line, thus it is
most natural to insert a suggestion about this option there.
Merge pull request #335 from rolk/335_multiver

Support multi-versioned installation