Make ALUGRID_DISABLE_METIS a proper CMake option

This MR changes the way METIS is disabled inside of the dune-alugrid module. It simply disables finding it. This disabling is module-local and does not influence other downstream modules. Since this decision influences the dunealugrid library but is also local to alugrid, a new entry in the config.h file is added to indicate this.

Details

The effect of setting variable ALUGRID_DISABLE_METIS was before this MR that internally the HAVE_METIS CMake varibale was unset. But HAVE_METIS is not checked anymore after the the dune-common macros are parsed. It is not (and was never) meant to be a customization point for enabling or disabling METIS. The HAVE_METIS CMake variable is actually set once and then considered a readonly property that modules can check but a modification has no effect at all.

An alternative to disabling that CMake searches for METIS is to not set the METIS flags to the dunealugrid library.