CMake: Demote the mixed linkage check to warning

Author: ywwry66

cmake/system.cmake

@@ -418,10 +418,14 @@ if (USE_OPENMP)
   if (NOT NOFORTRAN)
     find_package(OpenMP COMPONENTS Fortran REQUIRED)
     # Avoid mixed OpenMP linkage
-    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.")
+    get_target_property(OMP_C_LIBS OpenMP::OpenMP_C INTERFACE_LINK_LIBRARIES)
+    get_target_property(OMP_F_LIBS OpenMP::OpenMP_Fortran INTERFACE_LINK_LIBRARIES)
+    if (NOT OMP_C_LIBS STREQUAL OMP_F_LIBS)
+      message(WARNING
+	"OpenMP runtimes detected:\n"
+	" C=${OMP_C_LIBS},\n"
+	" Fortran=${OMP_F_LIBS}.\n"
+	"Please check that they are the same OpenMP runtime implementation.")
     endif()
   endif ()
 endif ()