emscripten: prefer -pthread over -s USE_PTHREADS=1

See: emscripten-core/emscripten#12346

This is supported since Emscripten 1.38.33, see:
emscripten-core/emscripten@2435079

mesonbuild/compilers/mixins/emscripten.py

@@ -59,10 +59,10 @@ def _get_compile_output(self, dirname: str, mode: str) -> str:
         return os.path.join(dirname, 'output.' + suffix)
 
     def thread_flags(self, env: 'Environment') -> T.List[str]:
-        return ['-s', 'USE_PTHREADS=1']
+        return ['-pthread']
 
     def thread_link_flags(self, env: 'Environment') -> T.List[str]:
-        args = ['-s', 'USE_PTHREADS=1']
+        args = ['-pthread']
         count: int = env.coredata.options[OptionKey('thread_count', lang=self.language, machine=self.for_machine)].value
         if count:
             args.extend(['-s', f'PTHREAD_POOL_SIZE={count}'])