This teaches the ExternalProject module to check the download URL file
name. If it is not a tarball (.tar, .tgz, .tar.gz) it is an error
because UntarFile does not yet understand other archive formats.
This creates new module ExternalProject.cmake to replace the prototype
AddExternalProject.cmake module. The interface is more refined, more
flexible, and better documented than the prototype.
This also converts the ExternalProject test to use the new module. The
old module will be removed (it was never in a CMake release) after
projects using it have been converted to the new module.
David Cole
Brad King