When building projects dependending on VTK, we ran into issues since some
variables were expected to be declared e.g. VTK_WRAP_PYTHON_INIT_EXE,
VTK_WRAP_HIERARCHY_EXE. These are unnnecessary since they are imported targets
and should be used if present. Updated code to use the targets directly if
found.

Change-Id: I74b5996671e195b02bd65b85b71a2e33bd3ca2da