Show libcurl's timestamp. This timestamp is only available in curlver.h

for tarball-based tests and builds, the maketgz script inserts it when
the tarball is created. For CVS-based tests and builds the timestamp we
show is the current UTC build time as it is the CVS version timestamp.

In this way, all builds will have a valid source code timestamp which
isn't related to the moment the tests and build is performed, with the
exception of CVS-based ones which have the same "date" and "timestamp"