[TLSRPT] An api version number please

Hi,

I'm looking at using libtlsrpt with exim https://bugs.exim.org/show_bug.cgi?id=3132

One thing I have come across several times is that packages are created, used in other projects, and then the first package is updated with new features, or (horror) incompatible changes. At this point a version number or similar is added to the package - *but* the older versions don't have a way for the consumer package to test the version being used.

It would be great if a header such as tlsrpt.h defined a variable such as TLSRPT_VERSION_NUMBER (or even a set of values and a macro, as in openssl/opensslv.h) *before* many distros package libtlsrpt.

Thanks,