gnutls: Disable parallel building

This can fail randomly with

CC systemkey.o
In file included from systemkey.c:47:0:
systemkey-args.h:1:1: error: unterminated comment
/* -*- buffer-read-only: t -*- vi: set ro:

which is presumably caused by a race with the generation step above:

autogen systemkey-args.def

http://hydra.nixos.org/build/37878764#tabs-buildsteps
(cherry picked from commit 024cb9eecd1f25dad64622ce1e59299059d5019c)

pkgs/development/libraries/gnutls/generic.nix

···
 
   # Build of the Guile bindings is not parallel-safe.  See
   # <http://git.savannah.gnu.org/cgit/gnutls.git/commit/?id=330995a920037b6030ec0282b51dde3f8b493cad>
-  # for the actual fix.
-  enableParallelBuilding = !guileBindings;
+  # for the actual fix.  Also an apparent race in the generation of
+  # systemkey-args.h.
+  enableParallelBuilding = false;
 
   buildInputs = [ lzo lzip nettle libtasn1 libidn p11_kit zlib gmp autogen nettools bash ]
     ++ lib.optional (stdenv.isFreeBSD || stdenv.isDarwin) libiconv