COMP: Fix configure error occurring when using XCode generator

Note that this fix only allow an "Archive" build to be successfully
done using Xcode. This could be done through the menu "Product" ->
"Build For" -> "Archiving"

Commit r21879 ensured python was systematically build in Release
on multi-configuration generator.

Since additional external project steps specific on unix and apple
system were not considered by the previous fix, it was causing
configure error like the one reported below:

../ExternalProject.cmake:1049 (add_custom_command):
add_custom_command given APPEND option with output
"/Volumes/Dashboards/Experimental/Slicer-Superbuild-Release-cmake-2.8.11-XCode/CMakeFiles/$(CONFIGURATION)$(EFFECTIVE_PLATFORM_NAME)/python-complete"
which is not already a custom command output.
[...]

This commit ensure that the additional steps consider the value of
"CMAKE_CFG_INTDIR" forced to Release.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22034 3bd1e089-480b-0410-8dfb-8563597acbee