[!367] fix CMAKE_GUARD condition for overlappingschwarztest

Merge branch 'bugfix/cmake_guard_condition_overlappingschwarztest' into 'master'

ref:core/dune-istl\> This MR fixes the CMAKE_GUARD condition for the
overlappingschwarztest.

See merge request [!367]

  [!367]: gitlab.dune-project.org/core/dune-istl/merge_requests/367

dune/istl/test/CMakeLists.txt

@@ -133,7 +133,7 @@ dune_add_test(SOURCES spqrtest.cc
               CMAKE_GUARD SuiteSparse_SPQR_FOUND)
 
 dune_add_test(SOURCES overlappingschwarztest.cc
-              CMAKE_GUARD SUPERLU_FOUND OR SuiteSparse_UMFPACK_FOUND)
+              CMAKE_GUARD "SUPERLU_FOUND OR SuiteSparse_UMFPACK_FOUND")
 
 dune_add_test(SOURCES cholmodtest.cc
               CMAKE_GUARD SuiteSparse_CHOLMOD_FOUND)