cpp was removed from /usr/libexec with FreeBSD 5.0-RELEASE.
Use __FreeBSD_version to tell imake if it's still there.
While here, X.Y.Z versioning ended with FreeBSD 3.0, so
we must be dealing with 2.Y.Z when checking for -lgnumalloc.
- also some rework of linux.cf
- only allow ELF systems
- use -pipe
- add some more defines
- get rid of some of the ancient (libc5/linux 1) support. Really,
don't expect CDE to build right on such old systems.
