different lib directories for gnutls and nettle

[Brian Gough]

Hi, now that GNU TLS is using nettle I notice there is a difference
between the --libdir for the two packages on 64 bit.

Nettle installs to $prefix/lib64/ and GNU TLS looks for it in
$prefix/lib/ -- so it fails and the standard configure/install to a
prefix doesn't work.

  (cd nettle-2.1 ; ./configure --prefix=/foo ; make ; make install)
  (cd gnutls-2.12.0 ; ./configure --prefix=/foo ; make ; make install)

  checking whether to use nettle... yes
  checking for libnettle... no
  configure: error: 
    ***
    *** Libnettle 2.1 was not found.

  make: *** [configure-work/gnutls-2.12.0/configure] Error 1

See http://chapters.gnu.org/~bjg/gsrc/logs/gnutls.txt for a complete
example.