build: refactor pkg-config for shared libraries

Improve detection and usage of pkg-config. This simplifies the setup
of all our shared libraries.

If pkg-config is installed on the host and `--shared` flags are passed
by the user, we try to get defaults from pkg-config instead of using the
default provided by configure.

PR-URL: https://github.com/nodejs/io.js/pull/1603


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>