Capitalized all CMake key words for consistency.
Ensure CMake v2.6 still writes DCMTKConfig.cmake.

The enhanced DCMTKConfig.cmake that was recently introduced relies on CMake
version 2.8.8 or higher and uses the new CONFIGURE_PACKAGE_CONFIG_FILE
command. This commit makes sure that older versions as the minimum CMake
version 2.6 required by DCMTK produce a DCMTKConfig.cmake that has at
least all the information it contained before the switch to
CONFIGURE_PACKAGE_CONFIG_FILE.