library clean-up: Don't build libNX_Xtst anymore. Use system's libXtst shared library. (Fixes ArcticaProject/nx-libs#32).

nxcompshad/X11/include/Xdamage_nxcompshad.h: Update FIXME statement. Work-around needs to persist until libX11 from X.Org gets used by nxagent.

 In the process of building nxagent against more and more system-wide installed
 X.org libraries, we come to the limit of including structs from this (bundled
 nx-X11) and that (system-wide X.Org) library.

 This commit introduces a clear namespace separation of headers provided by
 nx-X11 and headers provided by X.Org. This approach is only temporary as we
 want to drop all nx-X11 bundled libraries from nx-libs.

 However, for a while we need to make this separation clear and also ship
 some reduced fake X.Org headers that avoid pulling in libX* and libNX_X*
 symbols at the same time.

 This patch has been tested on Debian jessie and unstable and requires no
 overall testing on various distros and distro versions, as we finally will
 drop all libNX_X* libraries and build against X.org's client libs.

 For now, this hack eases our development / cleanup process.