SCons: Add method to detect Emscripten and use it for warnings config

Emscripten is LLVM-based so we want to follow the same logic. But we can't just
put it as a match in `methods.using_clang()` as that would mess with the
compiler version detection logic used to restrict old GCC and Clang releases.

methods.py

@@ -828,6 +828,10 @@ def using_clang(env):
     return "clang" in os.path.basename(env["CC"])
 
 
+def using_emcc(env):
+    return "emcc" in os.path.basename(env["CC"])
+
+
 def show_progress(env):
     import sys
     from SCons.Script import Progress, Command, AlwaysBuild