Document X11 requirements for building Qt.

[biagas]

We've received reports for problems building Qt, some of which may be due to missing required packages.
Some aspects needed for VisIt fail 'silently' during Qt's build and are only discovered when trying to build or package VisIt.

An xcbglintegrations error comes up when packaging VisIt. See SRE 192

The xcb-native-painting flag we pass to VisIt can cause Qt build to fail with this error:

ERROR: Feature 'xcb-native-painting' was enabled, but the pre-condition 'features.xcb-xlib && features.fontconfig && libs.xrender' failed.

Qt has a page documenting its X11 requirements

Should this link be part of our build_visit RTD page?

Should it be part of bv_qt error messaging if qt's configure or build fails?

[brugger1]

The actuall packages used on various systems are in the Dockerfiles, so referencing those would also be potentially helpful.

[biagas]

There is also this wiki which has more information about the libxcb requirements for various systems:
https://wiki.qt.io/Building_Qt_5_from_Git

Not sure if it is better to include links to the Qt wiki directly or pull the information out (in case the link changes in the future) and reference that the content came from the Qt pages.