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.
By using a separate variable for this we can easily grep for where
the build system depends on its own location, and also change the
policy regarding this later.
Note that there is no opm-macros module; this is a "psuedo"-module
which links to an additional one (currently, the one we're in)
