Detection based on gnutls_session doesn't work because GnuTLS 2.x.y
comes with a compat.h that defines gnutls_session to gnutls_session_t.

Instead detect this based on LIBGNUTLS_VERSION_MAJOR. Move this from
configure/config.h to gnutls_1_0_compat.h and make sure that all users
include gnutls_1_0_compat.h properly.

Also fix header guard in gnutls_1_0_compat.h.