Add a new FindGperf module

Summary:
Add a simple module to look for GNU gperf at build time, providing an
helper macro for adding generations to a list of sources.

gperf will be used to generate the C/C++ sources at build time, instead
of using static versions in VCS; at least kcodecs, khtml, and kio-extras
will be switched to this method.

Reviewers: Windows, Frameworks, Build System, kde-mac, adridg, rjvbb

Reviewed By: adridg, rjvbb

Subscribers: kfunk, rjvbb, adridg

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D3830