compatability: heirloom-utils +.5, libarchive -1

[Isaac Dunham <idunham@lavabit.com>]

I was building a kernel on Sabotage Linux (I'm using a rather modified
version already...), with initramfs enabled (which Sabotage does not
seem to support).  make would die because it couldn't find a cpio.xz
archive, so I assumed I needed a full cpio and xz.
That wasn't the issue, but I found some more compatability stuff:
xz: Seems to build fine (well, selecting {en,de}coders may not work
right, but otherwise it builds unchanged)
heirloom-utils: They are trying to support other nonstandard linux
libcs, so they check for __GLIBC__ | __uclibc__ to determine whether
dirent.h is present & sufficient; if dietlibc is detected, both
unistd.h & dirent.h are used.  The fallback is sys/dirent.h This should
probably be __linux__ | _POSIX_SOURCE triggers dirent.h inclusion, then
__diet__ triggers unistd.h; but if libc5/newlib are nonconformant here,
then that logic would be inadequate.
libarchive: needs memory.h --I just dropped it, since the syntax isn't
exactly like string.h, and there are at least two other cpio
implementations

Also, could PATH_MAX be exported unconditionally, or is that
nonstandard? Currently, defining any of the standard compatability
macros enables it.
This is needed for the kernel to build without defining HOSTCFLAGS,
though it really wants -D_GNU_SOURCE

Isaac