META_MODE: Fix 2nd build causing everything to rebuild due to changed CC.

In the first build the TOOLSDIR does not exit yet which causes CC to default
to the sys.mk version. Once a TOOLSDIR is created during the build though,
this logic was changing CC to ${TOOLSDIR}/usr/bin/cc even though that file
did not exist. Thus CC went from 'cc' to '/usr/bin/cc' which forced a
rebuild of everything while using the same compiler. Check that TOOLSDIR is
not empty to avoid this. If there is actually a TOOLSDIR cc then it will be
used and properly rebuild.

Sponsored by: EMC / Isilon Storage Division