New utility function Autom4te::ChannelDefs::merge_WARNINGS.

New utility function Autom4te::ChannelDefs::merge_WARNINGS.

This function merges a list of warnings categories into the environment
variable WARNINGS, returning a new value to set it to.  The intended use
is in code of the form

{
  local $ENV{WARNINGS} = merge_WARNINGS ("this", "that");

  # run a command here with WARNINGS=this,that,etc
}

This is not used yet, but will be in the next patch.

* lib/Autom4te/ChannelDefs.pm (merge_WARNINGS): New function.