Add support for install-able and relocatable package.

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

Considering that we still would like to support the CMake version 2.8.7
being provided into the Ubuntu 12.04 LTS, simply bumping up the minimum
CMake required version wouldn't work. Instead, the CMake
module CMakePackageConfigHelpers (corresponding to CMake kitware/CMake@f051814) has been
copied into CTK source tree and is included only if it is not already
provided by the CMake version being used to configure CTK. At configure
time status message will allow to understand which version of the module
is included. For example:

-- Including CMake built-in module CMakePackageConfigHelpers
-- Including CMake built-in module CMakePackageConfigHelpers - failed
-- Including CTK module CMakePackageConfigHelpers
-- Including CTK module CMakePackageConfigHelpers - ok


The CTKConfigVersion file is automatically generated with the
help of "write_basic_package_version_file" and is identical for both the
build and the install tree.


From CTKConfig availables in either the built or installed tree, remove
the following variables:
  CTK_EXECUTABLE_DIRS
  CTK_RUNTIME_LIBRARY_DIRS
  CTK_QT_RCC_EXECUTABLE
  CTK_QT_MOC_EXECUTABLE
  CTK_QT_UIC_EXECUTABLE
  CTK_MAJOR_VERSION
  CTK_MINOR_VERSION
  CTK_PATCH_VERSION

The following variables will now be specific to only the CTKConfig file
available in the *build* tree:
  CTK_QTDESIGNERPLUGINS_DIR
  CTK_DGRAPH_EXECUTABLE
  CTK_QT_QMAKE_EXECUTABLE
  CTK_EXTERNAL_LIBRARIES
  CTK_EXTERNAL_LIBRARY_DIRS

Add install rules for:
  UseCTK.cmake
  ctkConfig.h
  CTKPluginUseFile.cmake
  CTK targets
  CTKConfig.cmake
  CTKConfigVersion.cmake

See #403
Merge branch '403-relocatable-ctkconfig-usectk' into pu

* 403-relocatable-ctkconfig-usectk:
  Add support for install-able and relocatable package.
  Remove explicit setting of CMake policies.
  Explicitly set default value for CTK_INSTALL_QTPLUGIN_DIR
  Remove commented code and unused vars from CTKConfig / CTKGenerateCTKConfig
  Set plugin install rules for target and include files
  Install CTK's *.cmake files
  By default, do not install application headers
  Ensure "CMAKE_INSTALL_PREFIX" is properly passed to inner project.
  Ensure CTK_INSTALL_QTPLUGIN_DIR is properly set