Add support for reading the location of the native optimizer from EMSCRIPTEN_NATIVE_OPTIMIZER environment variable.

tools/js_optimizer.py

@@ -30,6 +30,8 @@ NATIVE_OPTIMIZER = os.environ.get('EMCC_NATIVE_OPTIMIZER') or '1' # use native o
 def get_native_optimizer():
   if os.environ.get('EMCC_FAST_COMPILER') == '0': return None # need fastcomp for native optimizer
 
+  if len(os.environ.get('EMSCRIPTEN_NATIVE_OPTIMIZER')) > 0: return os.environ.get('EMSCRIPTEN_NATIVE_OPTIMIZER')
+
   FAIL_MARKER = shared.Cache.get_path('optimizer.building_failed')
   if os.path.exists(FAIL_MARKER):
     shared.logging.debug('seeing that optimizer could not be built (run emcc --clear-cache or erase "optimizer.building_failed" in cache dir to retry)')