Adding code to use pkg-config on linux and MacPorts

[Elettronik]

Adding code to use pkg-config on linux and MacPorts, thanks to suggestion of @sodabrew.
This modification let gem find icu-i18n easier, and let track lib names.

[osbornebrook]

This allows the gem to install correctly for me with a macports-installed icu. Thanks @Elettronik.

[scunz]

Any chance this can be merged to master? It fixes compilation at least for gentoo-linux based systems.

[rbq]

I came here because git_statistics wouldn't install. Can't believe this code is sitting here unmerged for 5 months. :(

[pascalj]

Still no chance to merge this? It also fixes the installation on FreeBSD.

[isanosian]

Hey guys, any chance to get this merged? I'm using OSX and it fails to install the thing through MacPorts, so this code could be helpful. No chance to switch to HomeBrew so far, 'cause they're just doing much worse job than MacPorts team and lack many things I use.

@osbornebrook how do you install the gem with this code without having this PR merged?

[cooljeanius]

I would like to see this pull request merged sometime

[Elettronik]

Seem that main author is trying to embed libraries in next version. I think this is a bad idea, since on linux systems ICU is already a standard library and is found on near all system. On the Mac OS side, I cannost say anything, since I don't own one, but using a library outside the control of a central package system, seem to be prone to cause future problem. Feel free to comment the above, any new point of view is welcome.

[cooljeanius]

Seem that main author is trying to embed libraries in next version.

Does this mean in #66? That still hasn't been merged yet, and has merge conflicts just like this one; I think that this PR's merge conflicts would probably be easier to resolve, since it's a simpler PR...