Module mode CMake probe for opm-material

Apparently, since opm/opm-porsol#54 a bug in OpmFind has been uncovered; you cannot find a opm-material unless you specify `-Dopm-material_DIR=`, or there is a `Findopm-material.cmake` module for it. I haven't spent time tracking this one down. This changeset pulls a patch out of opm/opm-porsol#63 which I think will fix it; however, I haven't tested it. (If opm-porsol built for you before, it should also do so after this changeset)

@alfbr