Update Gentoo instructions to work with 17.1 and add missing dependency
Update Gentoo instructions to work with 17.1 and add missing dependency

 * libunwind was required when building.
 * Since Gentoo 17.1 the symlink between lib and lib64 has been removed.
    - Update export so it works with both older and newer profiles.

notriddle/servo,larsbergstrom/servo,saneyuki/servo,nnethercote/servo,notriddle/servo,paulrouget/servo,splav/servo,paulrouget/servo,KiChjang/servo,DominoTree/servo,paulrouget/servo,splav/servo,paulrouget/servo,nnethercote/servo,splav/servo,saneyuki/servo,splav/servo,DominoTree/servo,KiChjang/servo,saneyuki/servo,notriddle/servo,nnethercote/servo,larsbergstrom/servo,KiChjang/servo,larsbergstrom/servo,nnethercote/servo,splav/servo,splav/servo,larsbergstrom/servo,saneyuki/servo,KiChjang/servo,nnethercote/servo,nnethercote/servo,KiChjang/servo,notriddle/servo,larsbergstrom/servo,saneyuki/servo,DominoTree/servo,splav/servo,notriddle/servo,paulrouget/servo,paulrouget/servo,saneyuki/servo,larsbergstrom/servo,KiChjang/servo,paulrouget/servo,DominoTree/servo,KiChjang/servo,larsbergstrom/servo,larsbergstrom/servo,nnethercote/servo,DominoTree/servo,notriddle/servo,KiChjang/servo,notriddle/servo,saneyuki/servo,DominoTree/servo,splav/servo,DominoTree/servo,paulrouget/servo,notriddle/servo,nnethercote/servo,paulrouget/servo,saneyuki/servo,splav/servo,larsbergstrom/servo,KiChjang/servo,DominoTree/servo,saneyuki/servo,larsbergstrom/servo,KiChjang/servo,saneyuki/servo,DominoTree/servo,notriddle/servo,notriddle/servo,DominoTree/servo,splav/servo,nnethercote/servo,nnethercote/servo,paulrouget/servo