Support WWW variable in Makefile instead of the pkg-descr file.
This patch is based on review D34979 by @tcberner but with 2 noticeable changes:
- The www-site target is preserved and just returns the value of the WWW variable.
- The create-manifest script appends the value of the WWW variable to the copy of the pkg-descr file in the manifest.
Both these extensions of the patch in D34979 are meant to preserve previous behavior in case some 3rd party script depends on the target or structure of the pkg-descr file.
This patch-set should not be applied before there are WWW values in all port Makefiles.
The WWW variables in port Makefiles depend on updates of the portlint and portfmt ports (sent to the respective maintainers) to be committed to the ports tree.