Fix configuration when parmetis is not found

cmake/FindParMETIS.cmake

@@ -7,16 +7,16 @@ if (EXISTS "${PARMETIS_INCLUDE_DIR}/parmetis.h")
   message("-- Found parmetis.h in ${PARMETIS_INCLUDE_DIR}")
   find_library(LIB_PARMETIS parmetis)
   if("${LIB_PARMETIS}" STREQUAL "LIB_PARMETIS-NOTFOUND")
-    message(FATAL_ERROR "Could not find parmetis library")
+    message(WARNING "Could not find parmetis library")
   endif()
   find_library(LIB_METIS metis)
   if("${LIB_PARMETIS}" STREQUAL "LIB_METIS-NOTFOUND")
-    message(FATAL_ERROR "Could not find metis library")
+    message(WARNING "Could not find metis library")
   endif()
   set(PARMETIS_LIBRARIES ${LIB_PARMETIS} ${LIB_METIS})
   message("-- Found parmetis/metis libraries ${PARMETIS_LIBRARIES}")
 else()
-  message(FATAL_ERROR "Could not find parmetis.h")
+  message(WARNING "Could not find parmetis.h")
 endif()
 
 mark_as_advanced(PARMETIS_INCLUDE_DIR PARMETIS_LIBRARIES)