Replace non-portable (on MacOS X at least) `sed` with a `grep -v`
Replace non-portable (on MacOS X at least) `sed` with a `grep -v`

This fixes `oasis setup` and compiling the development version of Lwt
for me on MacOS X 10.8.

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