Modern CMake for scitokens-cpp

[bbockelm]

As suggested by @amadio in a discussion on the xrootd GitHub tracker (see xrootd/xrootd#2453), it'd be useful to modernize the CMake setup for scitokens-cpp. Items that come to mind:

• Installing CMake definitions from scitokens-cpp as part of the RPM install.
• Exporting build targets to make building and linking against scitokens-cpp easier.
• Exporting a version number for CMake.

@amadio, I roughly know how to do the first two items. Any suggestion on the best way to accomplish the third? When I've done things like that in the past, they've ended up poorly as we always seem to forget to update the code before tagging (it relied on humans to get the versioning correct and humans are poor at process).

[amadio]

In XRootD I did it in such a way as to automatically convert the tag into a version, see https://github.com/xrootd/xrootd/blob/master/cmake/XRootDVersion.cmake. You can use that pretty much verbatim, just changing the name of the project, then generate a version header like we do in XRootD at config time.