Dependency on cairo and xcb-shm

[johnmarkos]

I attempted to install brain using npm install, and I got the following error.

Perhaps you should add the directory containing `cairo.pc'
to the PKG_CONFIG_PATH environment variable
No package 'cairo' found

So, I attempted to install cairo, but there also appears to be a dependency upon xcb-shm. I think this may be cairo's dependency. In my case, on a Mac, I attempted to install cairo using homebrew.

At any rate, perhaps the installation instructions could be expanded to include cairo installation, since it's non-obvious? I'll be happy to send a PR if I get it working, but if somebody has already done this successfully, I'd really appreciate some pointers.

[johnmarkos]

After installing cairo on a Mac with brew install, I was able to get npm install to work with brain.js by setting

export PKG_CONFIG_PATH=/usr/local/lib/pkgconfig:/opt/X11/lib/pkgconfig

as described here.

[MatthewClair]

Cairo seems to only be used for the cross validation test. You don't need cairo outside running the tests. If you run npm install --production it will skip the dev dependencies.