ChannelDefs

[Akim Demaille]

The fact that Channels is parameterized is a Good Thing, but I still
would like to share ChannelDefs between Automake and Autoconf.  First,
it simplifies the maintenance (which is my major point), but will also
help us stay synch on the warning categories (this is a huge benefit
for both autoreconf and the user).  The default will be that maybe
some messages will be less application specific.

I would like to suggest that:

  `gnu'           GNU coding standards (default in gnu and gnits modes)

maybe the parentheses are not truly useful here, and it would weird in
autoreconf (but it makes sense).

  `obsolete'      obsolete features or constructions
  `override'      user redefinitions of Automake rules or variables

Hum...  Do we need to keep a reference to Automake here?  Again, not a
big problem with me if it stays here.

  `portability'   portability issues
  `syntax'        dubious syntactic constructs (default)
  `unsupported'   unsupported or incomplete features (default)
  `all'           all the warnings
  `no-CATEGORY'   turn off warnings in CATEGORY
  `none'          turn off all the warnings
  `error'         treat warnings as errors

I would also like to add

  `cross'         cross compilation issues