In libdockapp 0.7.0, dockapp.h was moved to /usr/include/libdockapp.
Obtained from [1] at the request of Nerijus Baliunas [2]. [1] https://web.archive.org/web/20110720231607/http://www.fcoutant.freesurf.fr/download/wmmenu-1.2.tar.gz [2] http://lists.windowmaker.org/dev/msg07991.html