Package version in PG_VERSION and version()

To be able to identify more easily which package a connected server is
coming from, I would like to embed the (Debian) package version in the
version() output which is coming from PG_VERSION. It is fairly easy to
do that, but it requires patching configure(.in):

$ ./configure VENDOR_VERSION="Debian 10.1-2"

# select version();
                                                      version
══════════════════════════════════════════════════════════════════════════════════════════════════════════════════
 PostgreSQL 10.1 on x86_64-pc-linux-gnu (Debian 10.1-2), compiled by gcc (Debian 7.2.0-17) 7.2.1 20171205, 64-bit

PoC patch against HEAD attached - if the approach is deemed
acceptable, I'll also update the relevant documentation bits.

Thanks,
Christoph