Log Message:
Mark this ONLY_FOR_PLATFORM and list the platforms where tcsh is *not*
already in /bin. Otherwise, building and installing it naively clobbers
the native tcsh, which isn't what anyone wants.

I believe these platforms to be: NetBSD, OpenBSD, Solaris -- if anyone
can confirm that others (especially old ones I can't readily check)
don't have a native tcsh, please add.