Remove USE_CAPABILITIES.

* configure.ac: Remove --with-libcap.
* curses/Makefile.am, efl/Makefile.am: Remove LIBCAP.
* emacs/Makefile.am, fltk/Makefile.am: Ditto.
* gnome3/Makefile.am, gtk+-2/Makefile.am: Ditto.
* qt/Makefile.am, tqt/Makefile.am: Ditto.
* tty/Makefile.am: Ditto.

Acquiring cap_ipc_lock (for mlock) from the process only made sence
when capabilities support was developed partially. Now, it should be
added (if needed) by file capabilities support or by inherited
capabilities.

For GNU/Linux, which enables a capability for mlock to normal users,
it's irrelevant, anyway.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>