ENH: Update elastix version

Fix an issue with compilation of ITK v4.8.1 on Mac OS X
Add CMakeLists.txt to root folder

SuperBuild/ExternalElastix.cmake

 set( proj Elastix )
 
 set( ELASTIX_REPOSITORY git://github.com/mstaring/elastix.git )
-set( ELASTIX_TAG 6eb6fc7e0d86bafdfdffb1b8bbd125f2a56a486e )
+set( ELASTIX_TAG c35842cd0152d8fd2894e7c6706d2dd8396f0fed )
 
 ExternalProject_Add( ${proj} 
   GIT_REPOSITORY ${ELASTIX_REPOSITORY}