The previous version assumed that we had libraries, and thus always
installs the .pc file in the multi-arch library directory. However,
we now have modules which does not have a library, but whose header
files still need to be located. Since the lib/ directory is usually
in the pkgconfig search path, it is natural to put them there.
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)
