gnutls: Build in parallel, cited problems were fixed upstream.

The systemkey problem was fixed in 3.4.15 [1].

The guileBindings issue was fixed 3 years ago, and is included
in all versions of gnutls we use today [2].

[1] 25d2f643c0
[2] 0d34b03f0e

pkgs/development/libraries/gnutls/generic.nix

@@ -34,11 +34,7 @@ stdenv.mkDerivation {
   ] ++ lib.optional guileBindings
     [ "--enable-guile" "--with-guile-site-dir=\${out}/share/guile/site" ];
 
-  # 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.  Also an apparent race in the generation of
-  # systemkey-args.h.
-  enableParallelBuilding = false;
+  enableParallelBuilding = true;
 
   buildInputs = [ lzo lzip nettle libtasn1 libidn p11_kit zlib gmp autogen ]
     ++ lib.optional doCheck nettools