- cd9660 relies on an #include "iso.h" but does not build any .c files
out of source, so remove reach-over .PATH
- ffs does not rely on any sys/ headers, so remove -I from CFLAGS.
- ffs_tables from sys/ is used by ffs; move the SRCS entry from the top-
level Makefile to ffs' Makefile.inc.
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