It's out of date and not included in the build. Instead, xf86DefModeSet.c is
built from vesamodes and extramodes using modeline2c.awk and *that's* what gets
built.
From bugzilla bug 13467ยน:
The modeline2c script is the only part of the Xorg server that requires Perl.
[This] is a simpler replacement that works with any normal AWK.
1] http://bugs.freedesktop.org/show_bug.cgi?id=13467
Bug was posted by Joerg Sonnenberger <joerg@NetBSD.org>.