New channel `portability-recursive'.

New channel `portability-recursive'.

Add new channel for portability warnings about recursive make
variable expansions `$(var1$(var2))'.  Enable it alongside
`-Wportability'.

* lib/Automake/ChannelDefs.pm (Automake::ChannelDefs): Register
channel `portability-recursive'.
* lib/Automake/Variable.pm (_VARIABLE_CHARACTERS)
(_VARIABLE_RECURSIVE_PATTERN): New variables.
(check_variable_expansions): Diagnose recursive variable
expansions through the new channel.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>