COMP: Fix build error when building on MacOSX 10.8/10.9. Issue #3405

This commit updates the python build system and ensure tk can be configured
when X11 header and libraries are in non-system location.

Python build system changes:

$ git shortlog 3c5864f2..47845 --no-merges
Jean-Christophe Fillion-Robin (12):
      Update .gitignore
      README: Re-organize README using markdown
      README: Associate Apache 2.0 License
      README: Update curl command to follow redirect
      README: Add instruction to check MD5 sum of the downloaded source package
      README: Add instruction to extract Python source archive
      README: Reference license file using relative link
      README: Format list of CMake options
      Display system processor
      Fix get_target_property warning related to non existent target
      Fix readline build error occurring when targeting MacOSX SDK 10.4
      Find X11 library and header to fix tkinter compile error. Fixes #33

Matt McCormick (3):
      Update Python version in README 2.7.1 -> 2.7.3.
      Support build for Linux/ARM.
      Reformat README from Markdown to reStructuredText.

Yann Collette (2):
      Simplify handling of python version introducing PY_VERSION CMake variable
      Fix failed uint{32,64}_t / int{32,64}_t detection on some platforms.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23608 3bd1e089-480b-0410-8dfb-8563597acbee