warnings: fix buglets for portability warnings

warnings: fix buglets for portability warnings

* lib/Automake/ChannelDefs.pm (switch_warning): Ensure the
correct implications and inter-dependencies between warnings
in the categories `portability', `extra-portability' and
`recursive-portability' are respected.  Also add detailed
explicative comments, and references to the relevant tests.
* tests/dollarvar2.test: Update and extend.  Also, remove
some unnecessary uses of `--force' option in automake calls.
* tests/extra-portability3.test: New test.
* tests/Makefile.am (TESTS): Add it.