Add a CMake warning for (mostly) old compilers.

CMakeLists.txt

@@ -21,6 +21,9 @@ endif()
 set(CMAKE_CXX_FLAGS_RELEASE "-Os")
 
 if((${CMAKE_CXX_COMPILER_ID} STREQUAL GNU) OR (${CMAKE_CXX_COMPILER_ID} STREQUAL Clang))
+    if (CMAKE_CXX_COMPILER_VERSION VERSION_LESS 11.0)
+        message(WARNING "A ${CMAKE_CXX_COMPILER_ID} compiler version of preferably 11 or higher should be used.")
+    endif()
     set(CMAKE_EXE_LINKER_FLAGS "-static-libgcc")
     set(CMAKE_CXX_FLAGS "-flto ${CMAKE_CXX_FLAGS} -fpie -pipe -fstack-protector-strong -fstack-clash-protection")
 endif()