[CMake] Don't add -lm to METIS_LIBRARY if this is not found.

cmake/modules/FindMETIS.cmake

@@ -48,7 +48,7 @@ find_library(METIS_LIBRARY ${METIS_LIB_NAME}
 )
 
 # we need to check whether we need to link m, copy the lazy solution from FindBLAS and FindLAPACK here.
-if(NOT WIN32)
+if(METIS_LIBRARY AND NOT WIN32)
   list(APPEND METIS_LIBRARY "-lm")
 endif()