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.