COMP: Remove commented/unused CMake code from External_ITKv4.cmake

See #1739

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22709 3bd1e089-480b-0410-8dfb-8563597acbee
COMP: Remove "-fopenmp" hack from External_ITKv4

This hack was first introduced while depending on ITK v4.0a07 (as
specified in Slicer r17596 comment). Following v4.0a07, ITK
build system has been updated to properly deal with the
issue (See kitware/ITK@8a135bcb).

Considering that
  (1) Slicer now depends on a version of ITK including
commit kitware/ITK@8a135bcb
  (2) variables "ITK_CMAKE_C_FLAGS" and "ITK_CMAKE_CXX_FLAGS" have
never been used to configure ITKv4 project
  (3) variable "CMAKE_CXX_FLAGS" was improperly referenced as "CMAKE_CX_FLAGS"

the corresponding hack is removed from the external project.

See #1739

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22710 3bd1e089-480b-0410-8dfb-8563597acbee
COMP: Explicitly set CTKDICOMCore as dependency of qSlicerBaseQTCore

Since qSlicerCoreApplication.h include ctkDICOMDatabase.h, this commit
add an explicit dependency between the two libraries.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22711 3bd1e089-480b-0410-8dfb-8563597acbee
COMP: Support Slicer_USE_SYSTEM_* option. Fixes #1739

<<<<< WORK IN PROGRESS - DO NOT INTEGRATE >>>>>>

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

This commit also add support for:
  -DSlicer_USE_SYSTEM_ITKv4:BOOL=ON
COMP: ExternalProject - Rename python variables with a slicer independent name
COMP: Set (tcl|tk)_LIBRARY variable into tcl external project