Assess variables to emit in generated config.gypi

[codebytere]

Assess which if any of the config variables are important to include in the js2c'd config.gypi generated by tools/generate_config_gypi.py.

/cc @nornagon

[hashseed]

I implemented a way to generate config.gypi using a Jinja2 template here to fix Node.js tests failures.

[nornagon]

Hm, jinja2 is an odd choice. config.gypi is just a python data structure, so repr() should be sufficient, no?

[hashseed]

I'm not a python guy. I just took it because we have it in DEPS anyways. I found replacing stuff in a template easier to read than programmatically altering an object like Node.js' configure script does.