x11-toolkits/qt5-gui: add X11 option

This change makes it possible to build Qt5 without a big dependency on X11,
which you might want to do when only requiring Qt for wkhtmltopdf or similar
"headless" setups.

• Some additional port formatting

PR: 247994
Submitted by: Ralf van der Enden <tremere@cainites.net>
Reported by: Markus Mann <freebsd-public@it64.de>