[WIP] Make sure libsass headers are read first

[saper]

In the case libsass is installed system-wide (e.g. the headers are in /usr/local/include) and node requires reading headers from the same path for the unbundled shared libraries like libuv, make sure -I/usr/local/include will not take precedence over our local bundles libsass headers.

[saper]

Yes. That is how it should be, but it does not work. Paths added from behind the condition will be merged in, but they get appended regardless of +

We need this because I want to propose node-gyp change to use potentially systemwide paths for libuv and such. If an older libsass is installed globally old header files will be used instead of our new ones.

[nschonni]

@saper I just flipped this to Draft since it had "WIP" in the title. If it's good to go, feel free to flip it back