warnings: 'extra-portability' category is not implied by '-Wall'

warnings: 'extra-portability' category is not implied by '-Wall'

This change is only intended for the 1.11.x series.  In automake
1.12, `-Wall' will be meant to enable `-Wextra-portability'.

* lib/Automake/ChannelDefs.pm (switch_warning): Do not enable
extra-portability warnings when `-Wall' is given.  Use ...
($have_extra_portability ): ... this new global variable.
Add a trailing "1;" to avoid spurious errors upon the importing
of this module.
* tests/extra-portability.test: Update and extend.
* NEWS: Update.