Add a variable EMSDK that points to the root of the Emscripten SDK.

emsdk

@@ -1688,6 +1688,10 @@ def construct_env(tools_to_activate, permanent):
       print('')
 
   env_vars_to_add = []
+
+  # A core variable EMSDK points to the root of Emscripten SDK directory.
+  env_vars_to_add += [('EMSDK', to_unix_path(emsdk_path()))]
+
   em_config_path = os.path.normpath(os.path.join(emscripten_config_directory, '.emscripten'))
   if not 'EM_CONFIG' in os.environ or to_unix_path(os.environ['EM_CONFIG']) != to_unix_path(em_config_path):
     env_vars_to_add += [('EM_CONFIG', em_config_path)]