This is for consistency with terminology in stdenv (and the terms
"hostDrv" and "buildDrv" are not very intuitive, even if they're
consistent with GNU terminology).
Either eigen changed the hash of the tarball, or I did not commit its latest hash.
I add boost 1.46 apart, because 1.44 does not work easily for freecad.
svn path=/nixpkgs/trunk/; revision=26151