realpath on AIX

I see that there was a report (and a workaround) for $(realpath foo//)
on AIX, but AIX realpath has bigger problems. Paths containing //..
will confuse AIX realpath causing it to return ENOENT, so Make's
realpath function returns nothing, e.g. with GNU Make 4.4.1

gcc119:~/tmp$ echo 'all: ; @echo ">>>" $(realpath
/home/jwakely/tmp/../tmp)' | gmake -f-
>>> /home/jwakely/tmp
gcc119:~/tmp$ echo 'all: ; @echo ">>>" $(realpath
/home/jwakely/tmp//../tmp)' | gmake -f-
>>>

I haven't been able to reproduce this with anything except "//.." but
there might be other bugs with realpath on AIX.