Enable building the PWG examples.

Enable building the PWG examples.

Original commit message from CVS:
* configure.ac:
* examples/Makefile.am:
* examples/pwg/Makefile.am:
* examples/pwg/extract.pl:
Enable building the PWG examples.
* docs/pwg/advanced-interfaces.xml:
Add URI interface stub.
* docs/pwg/advanced-types.xml:
* docs/pwg/other-autoplugger.xml:
* docs/pwg/appendix-porting.xml:
* docs/pwg/pwg.xml:
Add porting guide (mostly stubs), remove autoplugging (see ADM).
* docs/pwg/building-boiler.xml:
* docs/pwg/building-chainfn.xml:
* docs/pwg/building-pads.xml:
* docs/pwg/building-props.xml:
* docs/pwg/building-state.xml:
* docs/pwg/building-testapp.xml:
Update the building-*.xml parts for 0.9 changes. All examples
code blocks compile in examples/pwg/*.