GNUTLS: define new API types when installed library is too old

GNUTLS: define new API types when installed library is too old

This patch enables ngIRCd to use GNUTLS in really old versions, tested
with version 1.0.16, that don't define the "new" data types ending in
xxx_t. LIBGNUTLS_VERSION_MAJOR isn't defined there as well, so we use
it to test if we must define the new types on our own.