set target for darwin binary compat in skarnet.org packages

skalibs:
execline:
s6-dns:
s6-networking:
s6-portable-utils:
s6-rc:
s6:

The above software uses the target triplet from `cc -dumpmachine` as a
binary compatibility check. However, on darwin, the output includes the
darwin version number, which leads to build failures against a binary
skalibs package built a different version of darwin than the current
system.

Explicitly setting target ensures code can be compiled against a skalibs
binary built on a different version of darwin.

See http://www.skarnet.org/cgi-bin/archive.cgi?1:mss:623:heiodchokfjdkonfhdph

pkgs/tools/system/s6/default.nix

@@ -26,7 +26,9 @@ in stdenv.mkDerivation rec {
     "--with-lib=${execline}/lib"
     "--with-dynlib=${skalibs}/lib"
     "--with-dynlib=${execline}/lib"
-  ] ++ [ (if stdenv.isDarwin then "--disable-shared" else "--enable-shared") ];
+  ]
+  ++ (if stdenv.isDarwin then [ "--disable-shared" ] else [ "--enable-shared" ])
+  ++ (stdenv.lib.optional stdenv.isDarwin "--target=${stdenv.system}");
 
   preBuild = ''
     substituteInPlace "src/daemontools-extras/s6-log.c" \