gstreamer: Fix multiple outputs split

- Explicitly moving the files breaks them, because the wrappers
reference the files by absolute path. Also this automatically
moves the manpages to $dev as well.
- Need to explicitly set --exec-prefix since the pkgconfig file has
`toolsdir=${exec_prefix}/bin`, breaking totem:

http://hydra.nixos.org/build/34980617/nixlog/1/raw

````
checking for BACKEND_TEST... yes
checking GStreamer 1.0 inspection tool... no
configure: error:
Cannot find required GStreamer-1.0 tool 'gst-inspect-1.0'.
It should be part of gstreamer-1_0-utils. Please install it.

builder for ‘/nix/store/npq2ihlsdniv4j3wbyparq9byjxqdi15-totem-3.18.1.drv’ failed with exit code 1
````

While at it, enable parallel build.

pkgs/development/libraries/gstreamer/core/default.nix

···
   };
 
   outputs = [ "dev" "out" ];
+  outputBin = "dev";
 
   nativeBuildInputs = [
     pkgconfig perl bison flex python gobjectIntrospection makeWrapper
···
 
   propagatedBuildInputs = [ glib ];
 
+  enableParallelBuilding = true;
+
+  preConfigure = ''
+    configureFlagsArray+=("--exec-prefix=$dev")
+  '';
+
   postInstall = ''
     for prog in "$out/bin/"*; do
         wrapProgram "$prog" --prefix GST_PLUGIN_SYSTEM_PATH : "\$(unset _tmp; for profile in \$NIX_PROFILES; do _tmp="\$profile/lib/gstreamer-1.0''$\{_tmp:+:\}\$_tmp"; done; printf "\$_tmp")"
···
   '';
 
   preFixup = ''
-    moveToOutput "bin" "$dev"
     moveToOutput "share/bash-completion" "$dev"
   '';