Add missing DEFINED in CMakeLists.txt

CMakeLists.txt

@@ -843,7 +843,7 @@ if (MPI_FOUND)
 else()
   if (PUGS_ENABLE_MPI MATCHES "^OFF$")
     message(" MPI: explicitly deactivated!")
-  elseif((NOT PUGS_HAS_PARMETIS) and (NOT PUGS_HAS_PTSCOTCH))
+  elseif((NOT DEFINED PUGS_HAS_PARMETIS) and (NOT DEFINED PUGS_HAS_PTSCOTCH))
     message(" MPI: deactivated: ParMETIS and PTScotch cannot be found!")
   else()
     if (PUGS_ENABLE_MPI MATCHES "^(AUTO|ON)$")