[exim-dev] base64url (& base32hex) operators; build fixes

I've pushed to master a build fix for handling the hash split spreading
the dependencies upon OpenSSL headers; I handled the pkg-config cases,
it's possible that `src/EDITME` should be updated to note that CFLAGS
will need updating, not just TLS_INCLUDE.

I also updated for GnuTLS but that's untested.

This is bug 1906.

Per an idea on exim-users, I've added new expansion operators; this is
in the rfc4648 branch of the main repo. Since we're into RC I'm not
merging a feature addition myself. Please feel free.

Note that I've added test cases but not invoked them: recent system
reinstalls leave me currently without the test suite available. See
what happens with the build farm?

This adds base64url/base64urld to use the URL & Filename Safe Alphabet,
so that `/` does not appear in the results.

While in there, I added base32hex/base32hexd; I'm not too sure about
this, since we're using numbers-only in a way which isn't the normal
text-encoding system (eg, `0` encodes to empty string, etc). But since
in there, I went for it. I'm not wedded to this, we could strip it out
again. If this handled non-numbers then the use-case would be simple:
it preserves sort-order.

Shockingly, the reference for the alphabets is ... the RFC named in the
branch-name, RFC 4648.

-Phil