COMP: Ensure OpenSSL libs can be referenced with single-config generator

When building Slicer on windows, the OpenSSL libraries are
available for both Debug and Release in a single package.

This commit ensure that the library can also be referenced when building
on windows in the case a single config CMake generator like Ninja is used.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22832 3bd1e089-480b-0410-8dfb-8563597acbee
COMP: Tweak python external project to use CMAKE_CFG_INTDIR only when needed

This commit doesn't change the outcome of the current build process. It
simply re-organize the code to minimize confusion.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22833 3bd1e089-480b-0410-8dfb-8563597acbee