gnutls: check for alpn and ocsp in configure

Check for presence of gnutls_alpn_* and gnutls_ocsp_* functions during
configure instead of relying on the version number.  GnuTLS has options
to turn these features off and we ca just work with with such builds
like we work with older versions.

Signed-off-by: Marcus Hoffmann <m.hoffmann@cartelsol.com>

Closes #1204