[fix] Use USE_SYSTEM_python2 variable.

USE_SYSTEM_python is no longer used since the update of the
common-superbuild. Now USE_SYSTEM_python2 or USE_SYSTEM_python3 has to
be used.

CMakeLists.txt

@@ -129,10 +129,10 @@ if (WIN32)
   endif()
 endif()
 
-if(UNIX AND NOT DEFINED USE_SYSTEM_python)
-   # it is still possible to pass -DUSE_SYSTEM_python=False to cmake
-   message(STATUS "Defaulting USE_SYSTEM_python to True (was undefined and we are on UNIX)")
-   set(USE_SYSTEM_python True CACHE BOOL INTERNAL)
+if(UNIX AND NOT DEFINED USE_SYSTEM_python2)
+   # it is still possible to pass -DUSE_SYSTEM_python2=False to cmake
+   message(STATUS "Defaulting USE_SYSTEM_python2 to True (was undefined and we are on UNIX)")
+   set(USE_SYSTEM_python2 True CACHE BOOL INTERNAL)
 endif()
 
 function (superbuild_add_packaging)