COMP: Use PATCH_COMMAND to set python project patch step

Using PATCH_STEP was preventing python from being configured, using
PATCH_COMMAND is the only documented way of associating a patch step with
an external project.

See http://www.cmake.org/cmake/help/v2.8.12/cmake.html#module:ExternalProject

See #3158

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22830 3bd1e089-480b-0410-8dfb-8563597acbee
COMP: Python ExternalProject - Fix configure warnings, consistent symlinks

$ git shortlog 892c95b5..82e1494
Jean-Christophe Fillion-Robin (3):
      Merge branch '26-create-installed-python-executable-symlinks'
      Escape double-quote in ConfigureChecks to fix CMake 2.8.12.1 warnings.
      Merge branch '32-fix-cmake2.8.12.1-configure-warning'

Will Dicharry (1):
      Create versioned symlinks on install to match autoconf python install.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22831 3bd1e089-480b-0410-8dfb-8563597acbee
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
COMP: Fix parallel build associating patch step with "python-source" project

It is not clear why associating the PATCH_COMMAND with the python
project caused issue when doing parallel build using Ninja on Windows.
The patch step was executed before the download of python could finish.

This commit "resolves" the problem by associated the patch command
with the python source project.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22834 3bd1e089-480b-0410-8dfb-8563597acbee