Fix for [pgpool-hackers: 1630] autoconf broken

Fix for [pgpool-hackers: 1630] autoconf broken

The autoconf was failing because of the missing docbook macro (.m4) files
mistakenly left out by the sgml documentation commit.

Apart from the autoconf fix, the commit also adds a new 'make docs' build target
for building the documentation and excludes the doc directory from the default
build targets. i.e. now 'make all' will not build the documentation