That's not actually true (any more); util-linux/fdisk_osf.c, whilst
Linux-specific, does use MAXPATHLEN, and networking/traceroute.c uses
MAXHOSTNAMELEN. These could be changed to use {PATH,HOST_NAME}_MAX, but it's
highly likely that a future change will re-introduce a use of one of those
macros and break the build on the Hurd, so why not just define them?