Add a way to prevent the Module from being exported globally.

In our application, we stuff emscripten into a closure in pre- and post-
JS files, and export our interface without exposing the Module that we
use to implement some of it.
Well, it's processed by your parseTools.js (not standard cpp) and the comparisons it supports are very limited. It takes the whole line as a string, then tokenises it using `str.split(' ')` without stripping or interpreting quotes in any way, so the comparison here does work.

Setting `EXPORT_NAME` to empty would be possible, and should be doable with normal build systems (I think I was initially cautious because it's a pain to pass empty arguments through cmake, but using the `-s EXPORT_NAME=\'\'` would be OK).

Where would be the best place to add a test for this argument? I'll have a hunt and add one.