gtk-doc: missing itstool on darwin

Submitted by Ilya Konstantinov

Link to original bug (#748433)

Description

There's no recipe for itstool and gtk-doc fails to build.

Workaround:
brew install itstool
echo /usr/local/opt/libxml2/lib/python2.7/site-packages >> /usr/local/lib/python2.7/site-packages/libxml2.pth

^ second line is according to the message that appear during libxml2 installation:

If you need Python to find bindings for this keg-only formula, run:
echo /usr/local/opt/libxml2/lib/python2.7/site-packages >> /usr/local/lib/python2.7/site-packages/libxml2.pth