Old versions of autoconf (before 2.60) did not include
support for the docdir and abs_builddir variables. This
patch emulates support for them.

Fixes https://fedorahosted.org/sssd/ticket/422

Right now, the pkg-config checks for the system version of
libdhash are forcibly disabled, requiring the SSSD to build it
from its own tree. In the future, when we split the libraries off
from the SSSD, it will be easy to switch this check to the
external library.

Also pass a flag to the delete callback to tell it if this is a normal
entry removal or we are cleaning up the tbale definitively.

Previously we were only building them but not running them.

Necessary for RPM builds on RHEL5