META MODE: Fix changing what "MACHINE=host" means when computing dirdeps for include/.

The _SKIP_BUILD is used while computing DIRDEPS. If MACHINE=host is passed in
then this logic was replacing 'MACHINE' with a literal value of the host arch,
which then caused the dirdeps graph to be wrong since it no longer had the
literal 'host' for any of include's dependencies.

This is a NOP currently since include/ is not usually built with MACHINE=host.

Sponsored by: EMC / Isilon Storage Division