This is in preparation of fixing #3137. The plan is not to use
libfile's MIME type detection but Qt's. As both mkvmerge and the GUI
need to auto-detect the MIME type, this means that the command line
applications will have to be linked against Qt's core library (but not
the GUI libraries).
Prevents using the system qmake6 in cross-compilation situations.
Also cleans up the paths to `lconvert`, `moc`, `rcc` and `uic` from
the Qt 6 detection so that Qt 5 can detect their own version.
Part of the implementation of #3115.
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.
