[qt5-build.eclass] Extend QT5_GENTOO_CONFIG syntax.

A single '!' as first element of a triplet always evaluates as false.
[dev-qt/qtgui:5] More QT5_GENTOO_CONFIG.

Package-Manager: portage-2.2.10
[dev-qt/qtgui:5] Handle xinput2 too.

Package-Manager: portage-2.2.10
[dev-qt/qtwidgets:5] Implement QT5_GENTOO_CONFIG.

Package-Manager: portage-2.2.10