Fix PKG_CONFIG_PATH on macOS

[hansottowirtz]

On macOS, PKG_CONFIG_PATH is always overridden on install. I have libffi and other packages installed elsewhere and they're already on PKG_CONFIG_PATH. In this fix, /opt/X11/lib/pkgconfig is added to the path instead of overriding it.

Typical error:

Package libffi was not found in the pkg-config search path.
Perhaps you should add the directory containing `libffi.pc'
to the PKG_CONFIG_PATH environment variable
Package 'libffi', required by 'gobject-2.0', not found

[lastw]

This repo is not maintained anymore.

You can try prebuilt node-rsvg: https://github.com/f3lang/node-rsvg-prebuilt, and there is similar PR: f3lang#12

@f3lang, i'm going to make this repo read-only. What do you think?

[hansottowirtz]

Okay, please also create an unmaintained note in the readme.