Add additional pkg-config support

[hughmcmaster]

ICU is detected through pkg-config nowadays.

The second commit adds support for detecting libxslt via pkg-config as well.

Almost all of the complex detection code should be removed, as the functions looked for have been around for many years and pkg-config is very much preferred over the legacy foo-config scripts. I’m happy to make these changes if you like.

[dajobe]

This patch fails in Travis CI when compiled with c++ which I used as a more advanced C compiler check. Something related to templates

[hughmcmaster]

I saw you'd fixed the templates issue.

I've rebased my patches on the master branch and fixed an error I found in my libxslt patch.

All Travis CI builds pass now.

[dajobe]

Looks great now thanks