[Cleanup] Check for alugrid

src/explicitgrid.hh

 #ifndef SRC_EXPLICITGRID_HH
 #define SRC_EXPLICITGRID_HH
 
+#if !HAVE_ALUGRID
+#error ALUGRID is required
+#endif
+
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wignored-qualifiers"
 #include <dune/grid/alugrid.hh>