Exported cmake config should be called ptex-config-version.cmake

[Larry Gritz]

ptex-version.cmake isn't the name that CMake fill look for, so relying on the current set of config files doesn't quite work, the version won't be detected. As far as I can tell, it's not enough that ptex-config.cmake "includes" the version file. (CMake won't do the right things in the right order with the PACKAGE_VERSION.)

Also, side note, in ptex-config.cmake, those find_package calls ought to be find_dependency, which is customary for the exported config files and the semantics are slightly different.