COMP: Update ExternalProjectDependency based on commontk/Artichoke@82fa9f4

This update (more specifically commit 82fa9f4: Set "use system" variable if it
has NOT already been explicitly set) should allow to build Slicer expecting
NUMPY to be on the system while requiring the CTKAppLauncher to be built
by Slicer.

The following configuration will now succeed:

$ cmake -DQT_QMAKE_EXECUTABLE:FILEPATH=/path/to/bin/qmake \
-DSlicer_USE_SYSTEM_NUMPY:BOOL=ON \
-DSlicer_USE_SYSTEM_CTKAPPLAUNCHER:BOOL=0 \
-DSlicer_USE_PYTHONQT_WITH_TCL:BOOL=OFF ../Slicer

See http://www.slicer.org/slicerWiki/index.php/Documentation/Labs/PythonCondaBuild#Different_approach

$ git shortlog c3eb170..82fa9f4 --no-merges
Jean-Christophe Fillion-Robin (12):
Rename Artichoke into ExternalProjectDependency
Add cmake sphinx extension based on Kitware/CMake@3c8226e59
Add explicit support for function and macro directive
Add sphinx gitignore
Add sphinx documentation
Convert README into rst and reference it from index.rst
Include "indices and tables" only if generating html
Add docs badge
Fix tests including the correct module (broken after commit 9106d3d)
Tweak test in ExternalProject_Message to avoid warning related to CMP0054
Update tests setting policy CMP0054 to OLD.
Set "use system" variable if it has NOT already been explicitly set

Matthew Woehlke (1):
Add function to install external CMake projects

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24001 3bd1e089-480b-0410-8dfb-8563597acbee