cmake: do not use -mmitigate-rop on GCC >= 9.1 /monero#5613

It was removed, but it still accepted by the compiler, which warns
for every file

CMakeLists.txt

@@ -684,8 +684,11 @@ else()
     add_cxx_flag_if_supported(-fstack-clash-protection CXX_SECURITY_FLAGS)
   endif()
 
-  add_c_flag_if_supported(-mmitigate-rop C_SECURITY_FLAGS)
-  add_cxx_flag_if_supported(-mmitigate-rop CXX_SECURITY_FLAGS)
+  # Removed in GCC 9.1 (or before ?), but still accepted, so spams the output
+  if (NOT (CMAKE_C_COMPILER_ID STREQUAL "GNU" AND NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 9.1))
+    add_c_flag_if_supported(-mmitigate-rop C_SECURITY_FLAGS)
+    add_cxx_flag_if_supported(-mmitigate-rop CXX_SECURITY_FLAGS)
+  endif()
 
   # linker
   if (NOT WIN32)