Revert "Remove extra docdir variable from Makefile.am"

This reverts commit 397, as the support in autoconf for docdir is
present since 2.59c, and this is too new for some distributions.

Reviewed-by: imsnah

doc/Makefile.am

+docdir = $(datadir)/doc/$(PACKAGE)
+
 SUBDIRS = examples
 dist_doc_DATA = \
   hooks.html hooks.pdf \