- Modify makefile to support GNU convention makefile variable names,
including prefix, bindir, datarootdir, mandir, and man1dir.
It is tweaked so that the older variable names (e.g., INSTALL_DIR)
will continue to work.
- DESTDIR support added.
- Predefined a useful INSTALL_PROGRAM makefile variable; that means
that users can redefine it if they want.