Explicitly enable fontconfig for qt5-base

[drbenmorgan]

Issue reported on Falaise whereby Qt GUI did not display did not display fonts:

https://github.com/SuperNEMO-DBD/Falaise/issues/6

On Linux only, explicitly depend on brewed fontconfig, which also pulls in freetype. Add needed fontconfig and system freetype args to configure flags on Linux. Mac installs will use qt's freetype.

Tested on macOS Sierra and CentOS 6.9. Requires checks on Ubuntu.

• [x] Have you followed the guidelines for contributing?
• [x] Have you checked that there aren't other open pull requests for the same formula update/change?
• [x] Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• [x] Does your build pass brew audit --strict <formula> (after doing brew install <formula>)?

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[drbenmorgan]

Now tested on Ubuntu 16.04, so merge..