[PATCH] CMake: add support for GNU/Hurd

[PATCH] CMake: add support for GNU/Hurd

Add a "HURD" CMake platform specification, so it can be properly
checked in the build system.

Set QT_DEFAULT_MKSPEC to the existing hurd-g++ mkspec.

Hurd supports $ORIGIN in RPATH, so enable it.

Hurd uses X11, so add it to the X11_SUPPORTED list.

Enable few more feature checks that apply to Hurd as well: either
because they are provided by GNU libc itself, or because they are
implemented on Hurd.

Check and set the ELF interpreter, as it is a common functionality of
the GNU toolchain.

Change-Id: Id347033560bbc5a2a4e2c3abb493c948c002b40e
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
Gbp-Pq: Name upstream_CMake-add-support-for-GNU-Hurd.patch