Allow setting --docdir
This will allow setting `--docdir` during configure, this is useful because not all linux distributions install documentation to `/usr/share/doc`. For example in Slackware documentation is installed to `/usr/doc/$PRGNAM-$VERSION` and `/usr/share/doc` is a symlink to `/usr/doc`.
To use this `./configure --docdir=/usr/doc/$PRGNAM-$VERSION` can be used.