#1616 configure sets HAVE_STRUCT_DIRENT64 when readdir64 unavail

[David Dyck]

After running configure in tcl8.4.5 and tk8.4.5 I get a
link error
at the end of make because readdir64 is not defined.

I edited the Makefile and tkConfig.sh (tclConfig.sh on
the tcl side)
to remove the HAVE_STRUCT_DIRENT64 and was able to
complete the make/install (and run tkdiff as a
testcase)

I'm using linux 2.4.23-rc with libc5.4.44
gcc version egcs-2.91.66.1