CMake: Demote the mixed linkage check to warning

Author: ywwry66

cmake/system.cmake

@@ -421,7 +421,7 @@ if (USE_OPENMP)
     get_target_property(OMP_C_LIB OpenMP::OpenMP_C INTERFACE_LINK_LIBRARIES)
     get_target_property(OMP_Fortran_LIB OpenMP::OpenMP_Fortran INTERFACE_LINK_LIBRARIES)
     if (NOT OMP_C_LIB STREQUAL OMP_Fortran_LIB)
-      message(FATAL_ERROR "Multiple OpenMP runtime libraries detected. Mixed OpenMP runtime linkage is dangerous. You may pass -DOpenMP_LANG_LIB_NAMES and -DOpenMP_omp_LIBRARY to manually choose the OpenMP library.")
+      message(WARNING "Different OpenMP runtime libraries might have been set for OpenBLAS's C and Fortran components. It is recommended to link to a single OpenMP library. You can manually set the OpenMP_C_LIB_NAMES, OpenMP_Fortran_LIB_NAMES and OpenMP_omp_LIBRARY variables.")
     endif()
   endif ()
 endif ()