Encourage the use of config mode if available

If the directory which is specified in OPM_CORE_ROOT is a build tree (e.g. -DOPM_CORE_ROOT=~/opm/bld/opm-core in contrast with -DOPM_CORE_ROOT=/usr), then use the opm-core-config.cmake file which is located there to directly import the configuration done earlier instead of running Findopm-core.cmake to do the probes again. (This is really something that should work in CMake, but which doesn't without some help)