ISC has merged in LDAP support as of dhcp-4.2.0.  From the RELNOTES
file:

- "The LDAP Patch" that has been circulating for some time, written by
  Brian Masney and S.Kalyanasundraram and maintained for application to
  the DHCP-4 sources by David Cantrell has been included.  Please be
  advised that these sources were contributed, and do not yet meet the
  high standards we place on production sources we include by default.
  As a result, the LDAP features are only included by using a compile-time
  option which defaults off, and if you enable it you do so under your
  own recognizance.  We will be improving this software over time.
  [ISC-Bugs #17741]



I will keep this project alive on github.com for users wanting to use
older releases of ISC dhcp and have LDAP support.  I cannot guarantee
that we will do a lot of heavy work on supporting those old releases.

Many thanks to everyone who has used this patch, contributed fixes, and
urged ISC to merge it upstream!