Unified - Add the build.info command OVERRIDE, to avoid build file clashes

Should it be needed because the recipes within a RAW section might
clash with those generated by Configure, it's possible to tell it
not to generate them with the use of OVERRIDES, for example:

    SOURCE[libfoo]=foo.c bar.c

    OVERRIDES=bar.o
    BEGINRAW[Makefile(unix)]
    bar.o: bar.c
        $(CC) $(CFLAGS) -DSPECIAL -c -o $@ $<
    ENDRAW[Makefile(unix)]

Reviewed-by: Rich Salz <rsalz@openssl.org>