Fix configuration of ctypes extension in case system processor is i686

This commit should fix the following configuration error:

// ---------------
CMAKE_SYSTEM_PROCESSOR:i686

/home/thiele/Slicer-SuperBuild-Debug/python-build/../Python-2.7.3/Modules/_ctypes/libffi/src/i686/ffi.c

  Tried extensions .c .C .c++ .cc .cpp .cxx .m .M .mm .h .hh .h++ .hm .hpp
  .hxx .in .txx
Call Stack (most recent call first):
  cmake/extensions/CMakeLists.txt:183 (add_python_extension)

// ---------------
Merge branch 'fix-ctypes-build-on-i686-system'

* fix-ctypes-build-on-i686-system:
  Fix configuration of ctypes extension in case system processor is i686