Look first for qhelpgenerator-qt5 for help generation

Summary:
A number of distributions uses the -qt5 suffix for Qt command line
tools, to make sure they can coexist with the Qt4 equivalents.

This change uses distutils's find_executable to look for the -qt5
suffix of qhelpgenerator. If it is not found, it falls back to the
un-suffixed version.

Reviewers: Frameworks, ochurlaud

Reviewed By: ochurlaud

Subscribers: fvogt, Documentation

Tags: Frameworks, Documentation

Differential Revision: https://phabricator.kde.org/D11315