COMP: Fix double-convert external project so that it works as intended

1) The previous version was working because the double-conversion project
was exporting its build tree in the cmake package registry.

See http://www.cmake.org/cmake/help/v2.8.10/cmake.html#command:export

Since the package was exported, the find_package(double-conversion ..) within
BRAINSTools was working. Line like the following weren't not used at all:

list(APPEND ${CMAKE_PROJECT_NAME}_SUPERBUILD_EP_VARS ${extProjName}_DIR:PATH)

2) Remove dependency to DCMTK. See https://github.com/BRAINSia/BRAINSTools/issues/5

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21791 3bd1e089-480b-0410-8dfb-8563597acbee