Only when configured, grant root access to the X11 display (#776437)

GParted fails to display when run under Wayland [1][2][3].  This is
because by intentional design Wayland doesn't allow applications with
root privileges access to the display [4].

As an interim workaround make the gparted shell wrapper use xhost to
grant root access to the X11 server if root doesn't already have access,
but only when configured.  Granting root access must be explicitly
enabled when building GParted like this:
    ./configure --enable-xhost-root
It defaults to disabled.  When gpartedbin binary ends the shell wrapper
revokes root access only if it granted such access.

[1] GNOME Bug 776437 - GParted fails to run as root under Wayland
    https://bugzilla.gnome.org/show_bug.cgi?id=776437

[2] Ubuntu Bug 1652282 - GParted does not work in GNOME on Wayland
    https://bugs.launchpad.net/ubuntu/+source/gparted/+bug/1652282

[3] Fedora Bug 1397103 - gparted not working under Wayland
    https://bugzilla.redhat.com/show_bug.cgi?id=1397103

[4] Common Fedora 25 bugs
    Running graphical apps with root privileges (e.g. gparted) does not
    work on Wayland
    https://fedoraproject.org/wiki/Common_F25_bugs#wayland-root-apps

Bug 776437 - GParted fails to run as root under Wayland