Finddune-grid.cmake: test for ALUGrid and METIS

ALUGrid is needed to avoid unresolved symbols if dune-grid was
compiled with it. METIS is used by the parallel version of
ALUGrid. (Although it is not open source...)
Add HAVE_CXA_DEMANGE config.h variable to Finddune-common.cmake

even if it is not tested, it will at least make Dune::classname usable
(on Dune master, the 2.2 release should be fine even without it).