Scons script doesn't allow compiling to javascript on windows

Steps to reproduce:
- install python, emscripten using chocolatey on windows 10
- run: scons platform=javascript

Result:
`raise ValueError("Required toolchain not found for platform " + env["platform"])`

Temporary fix: (javascript.py)
```
import os

def exists(env):
#return "EM_CONFIG" in os.environ
return True

def generate(env):
if env["arch"] not in ("wasm32"):
print("Only wasm32 supported on web. Exiting.")
Exit()

#if "EM_CONFIG" in os.environ:
env["ENV"] = os.environ
```