xorg build: include local settings if exist

Include xorg/rdp/LocalConfigSettings.cmake if exists to be
able to override settings like XOBJBASE_RELATIVE (required e.g.
when building for ubuntu)
X11R7.7 build: don't build python

Use the python interpreter installed on the system.
X11R7.7 build: removed not required dependencies

libxml2/libxslt
X11R7.7 build: removed not required dependencies

cairo
X11R7.7 build: added common configuration options

use COMMON_CONFIGURE_OPTS instead of setting --prefix (and similar) to
every external project seperately
X11R7.7 build: set X11RDP_PREFIX only if not already set

Allows to supplied X11RDP_PREFIX as cmake command line define
X11R7.7 build: split xlibs and fixed build dependencies

* Fixed build dependencies to make sure that libraries are available
when required for linking to prevent linking against system libraries
(for libs we build ourselves).
* Xlibs are now split into x11-base-libs (libs without dependencies)
and x11-libs.
X11R7.7 build: set libdir

Set --libdir when configuring an external project to CMAKE_INSTALL_LIBDIR.
to override the default (lib) in case of 64 bit system or when multiarch
is used.
X11R7.7 build: build dependency added

libXtst depends on libxi
Merge pull request #1 from bmiklautz/X11R7_build

X11 r7.7 build fixes