How does emscripten generate additional warnings?

I'm new to emscripten and I have a question regarding the emscripten's implementation. When I compile a code using emscripten it introduces the following error:

`error: 'long long' is an extension when C99 mode is not enabled [-Werror,-Wlong-long]`

I understand since the `-Werror` is enabled it treats warnings as errors. The version of `emsdk` I used takes `Clang version 19.0.0` to compile a C program. However, when I use the same version of Clang to compile my code into x86-64, no warning will arise. Can you help me to understand what is happening in this case? I supposed that emscripten uses Clang to compile code into LLVM IR. But how does it generate additional warnings which are not introduced by Clang?