[buildsystem] Call check for Boost base.

The check gets removed from dune-common, thus ISTL has to call it itself.

cmake/modules/DuneIstlMacros.cmake

+include(DuneBoost)
 find_package(BoostFusion)
 find_package(METIS)
 find_package(ParMETIS)

m4/dune_istl.m4

@@ -10,6 +10,7 @@ AC_DEFUN([DUNE_ISTL_CHECKS],
   AC_REQUIRE([__AC_FC_NAME_MANGLING])
   AC_REQUIRE([AC_PROG_F77])
   AC_REQUIRE([ACX_BLAS])
+  AC_REQUIRE([DUNE_BOOST_BASE])
   DUNE_BOOST_BASE(, [ DUNE_BOOST_FUSION ] , [] )
 
   # add summary entries for tests not maintained by dune