Make sure GNUTLS is first on uselib

Why? Because I have two versions of GnuTLS installed - one is old 2.0.X in
/usr and one is new 2.5.X in ~/local/gnutls. waf correctly finds that the
newer version, but because GNUTLS was behind other libraries in the
node.uselib -L/usr/lib was before -L/home/ryan/local/gnutls/lib in the
actual gcc command - hence getting link errors. WAF SUCKS, really.
I wish someone would invent a good build system that could avoid such
problems.