Qt 6
`configure` will look for Qt 6 first and only continue looking for Qt
5 if Qt 6 isn't found or disabled via `--disable-qt6`.
Qt 6 detection works by first looking for the `qmake6` binary. Its location
can be specified with the `--with-qmake6=…` option.
All other Qt 6 related facts (such as compiler & linker flags or the
position of the other required tools such as `lconvert`, `moc`, `rcc` and
`uic`) will be derived from the output generated by `qmake6`.
Note that at this point Qt 6 is not yet supported for the cross-compilation
build to Windows, nor is a static Qt 6 build supported yet.
Note that the command line options `--enable-static-qt`,
`--with-qt-pkg-config-modules` and `--without-qt-pkg-config` only apply to
the Qt 5 and have no effect on Qt 6.
Qt 5
The options for specifying the position of the tools (`--with-moc=…`,
`--with-rcc=…` and `--with-uic=…`) have been removed. Their position
will now be derived from the output generated by `qmake`.
General
Completely disabling the GUI now requires passing both `--disable-qt6`
and `--disable-qt` options.
Part of the implementation of #3115.
