Previously, in 683b7d7 and
0e75263, we accidentally broke QT's
ability to pick up Xlib thru the config.gui.tests.xlib configuration
test, which also means that config.gui.libraries.xcb_xlib wasn't run.

This resulted in a QT build that was implicitly -no-xcb-lib and
-no-feature-xlib.

This is actually a desired behaviour, as it means less required shared
objects for our final bitcoin-qt binary. Specifically, it eliminated the
libX11-xcb.so.1 and libX11.so.6 requirements.

In this commit, we explicitly build without Xlib. We should continue to
track upstream ticket https://bugreports.qt.io/browse/QTBUG-61452 which
talks about adding a -no-xlib (non-hidden) flag instead of the
-no-feature-xlib (hidden) flag.

We're no longer building QT with libX11/XLib, however, libX11/XLib
headers are still required for parts of QT. In this commit we add a
minimal configure.ac for libX11/XLib that is headers-only.

This change allows us to remove all of libX11/XLib's dependencies.

We're no longer building QT with libX11/XLib, so it doesn't make sense
to check for the x11-xcb package.

libXext was only needed (as a library) by QT when it was using
XLib/libX11 (as a library), now that we're building QT without
XLib/libX11, we can safely remove libXext.

They should no longer be needed as we build QT without libX11/XLib
libraries now.

There was a previous attempt to achieve this, and it was bad. This
works.

We can actually patch QT to remove its dependency on libX11's headers.
It turns it this wasn't that hard.