COMP: Support Slicer_USE_SYSTEM_* option. Fixes #1739

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On Ubuntu 13.10, the following options have been tested simultaneously:
  -DSlicer_USE_PYTHONQT_WITH_OPENSSL:BOOL=ON
  -DSlicer_USE_SYSTEM_python:BOOL=ON
  -DSlicer_USE_SYSTEM_DCMTK:BOOL=ON
  -DSlicer_USE_SYSTEM_LibArchive:BOOL=ON
  -DSlicer_USE_SYSTEM_curl:BOOL=ON

The associated system packages have been installed using:
  sudo apt-get install libssl-dev \
    libdcmtk-dev \
    libarchive-dev \
    libpython2.7-dev \
    libcurl4-openssl-dev