Add exists_if to META file to allow partial installation
Add exists_if to META file to allow partial installation

Ignore-this: cff70eca8e05d95dc7dd4e484c0a4af3

darcs-hash:20090513131920-0445d-f0c1b6e638b6b3b78ee1b12039d5a216f7093012

c-cube/lwt,rgrinberg/lwt,c-cube/lwt,fdopen/lwt,ocsigen/lwt,mirage/lwt,rgrinberg/lwt,mfp/lwt,ocsigen/lwt,rgrinberg/lwt,mfp/lwt,rneswold/lwt,ocsigen/lwt,mirage/lwt,rgrinberg/lwt,janestreet/lwt-async,gfxmonk/lwt,janestreet/lwt-async,fdopen/lwt,rneswold/lwt,gfxmonk/lwt,c-cube/lwt,rneswold/lwt