Continuation of commit 4e47b053c9d799.

All unittests still pass.

- We don't need the Threads target workaround anymore, that breaks the
build with Qt6 even.
- The Gradle wrapper shipped with Qt is no longer installed as executable,
so we need to run this in sh explicitly.
- Qt6 uses a different Android Gradle plugin version (not to be confused
with the Gradle version), which we need to make available for the
configure_file() call on the build.gradle file.

With this most Framework modules build against Qt6 here.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

SunOS and variants (Solaris, OpenSolaris, illumos, etc al.) do not need a separate library for inotify.

This doesn't make all these modules actually work with Qt6 yet, but it
prevents them from unconditionally pulling in Qt5 in a Qt6 build.

This is useful in order to even be able to pass a CMake run to the
point one can port, test and debug the rest.

PKG_CONFIG_SYSROOT_DIR only applies to -I and -L flags, so pkg-config
does not prepend CMAKE_SYSROOT when cross-compiling unless you pass
--define-prefix explicitly. Therefore we have to  manually do prepend
it here when cross-compiling.
See https://gitlab.kitware.com/cmake/cmake/-/issues/16647#note_844761.

With this change I no longer get errors related to wayland.xml when
cross-compiling KWaylandServer.

When cross-compiling QT_INSTALL_BINS contains binaries for the target
system that can't be used during the build. Instead use QT_HOST_BINS
which contains the Qt build tools for the current system.