Helpful error messages in build

CMakeLists.txt

@@ -49,6 +49,7 @@ if (OJL_BIMAP_BUILD_TESTS)
 
     if (NOT CXX_20_SUPPORT)
         message(FATAL_ERROR "The C++ compiler does not support the -std=c++20 flag")
+        message(FATAL_ERROR "Use a compiler that supports C++ 20, like clang++ >= 10")
     endif()