Don't include common.gypi from node source tree

[bnoordhuis]

node-gyp includes $nodedir/common.gypi when creating build files for an add-on.

Per nodejs/CTC#62 (comment), that probably isn't future-proof and it's a bad idea in general: it makes add-ons pick up compiler and linker settings that aren't intended for them and may be actively harmful. People have filed bug reports about it.

Plan of action:

1. Merge common.gypi into addon.gypi.
2. Strip everything that isn't needed by add-ons.
3. Test the bejebus out of it with citgm.

[murgatroid99]

I would support this change. I got burned by the fact that common.gypi explicitly disables -Werror

[richardlau]

Node.js 10's common.gypi requires C++14:
https://github.com/nodejs/node/blob/cf41627411886000429bde058a6594fb7f6d6d47/common.gypi#L324

which may expose anyone using older compilers to compile addons.

[bnoordhuis]

Yep, but that flows on down from V8. We'd still carry that change with a separate common.gypi.