Add generation of DCMTKConfig.cmake

***** NOTE: THIS COMMIT SHOULD BE SQUASHED WITH THE PREVIOUS ONE *****
*****       It hasn't been squashed to illustrate the change I   *****
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For both install and build tree, the files:

  DCMTKConfig.cmake
  DCMTKConfigVersion.cmake
  DCMTKTargets.cmake

will be configured into:

   <prefix>/share/dcmtk

It means the following option have to be passed when build a project
depending on python:

   -DDCMTK_DIR:PATH=/path/to/<prefix>/lib/cmake/dcmtk

Since CMake 2.8.8, there is convenient module named "CMakePackageConfigHelpers"
that provided the helper macros "configure_package_config_file" and
"write_basic_package_version_file" allowing to easily configure
relocatable packages.
See http://www.cmake.org/cmake/help/v2.8.8/cmake.html#module:CMakePackageConfigHelpers

To avoid upgrading the minimum requirement version of CMake, the
generation of 'DCMTKConfig.cmake' is done only if possible. To display
message with the macro are not available, a convenient
macro named "check_cmake_command_exists" has also been added.

To allow find_package to work with both a build and install tree, the macro
"configure_package_config_file" is called twice with different value for
the variables:

  DCMTK_CMKDIR_CONFIG
  DCMTK_config_INCLUDE_DIR_CONFIG
  DCMTK_<module>_INCLUDE_DIR_CONFIG

Then, within DCMTKConfig.cmake, the following variables are set:
  DCMTK_MODULES
  DCMTK_LIBRARIES
  DCMTK_TARGETS
  DCMTK_INCLUDE_DIRS
  DCMTK_INCLUDE_DIR # Deprecated

Finally, the DCMTKConfigVersion file is automatically generated with the
help of "write_basic_package_version_file" and is identical for both the
build and the install tree.



Remarks about "export(PACKAGE ...)" [1] statement:

It turns out that exporting the build tree in the system package registry
is a bad idea when building the same package multiple time as it is done
on a dashboard client or a developer workstation trying out multiple
configurations. Indeed, when "find_package" is used in "CONFIG"
(or "NO_MODULE") mode, by default CMake will search into the CMAKE_SYSTEM_PACKAGE_REGISTRY
and will resolve to an already generated build tree instead of the one
passed using <PROJECT_NAME>_DIR.


[1] http://www.cmake.org/cmake/help/v2.8.10/cmake.html#command:export