GnuTLS control constants exposed to Makefile.

Mostly care about EXIM_GNUTLS_LIBRARY_LOG_LEVEL for debugging.
If someone screams that we kept the default dh-bits at 1024 for old GnuTLS,
we can point them at EXIM_SERVER_DH_BITS_PRE2_12.  The name itself will
tell them to shut up and update their library if they care about security. :)