Our use of gnutls_rnd(), introduced with commit ad7520e8, is
conditional to the availability of the <gnutls/crypto.h> header
file.

Such check, however, turns out not to be strict enough, as there
are some versions of GnuTLS (eg. 2.8.5 from CentOS 6) that provide
the header file, but not the function itself, which was introduced
only in GnuTLS 2.12.0.

Introduce an explicit check for the function.