new functions in "make po-check"

[Gavin Smith]

There appear to be several new functions that have been added to
po/Makevars but not recognised by "make po-check" (rule in top-level
Makefile.am) to make a file contain translatable strings:

message_list_document_error
message_list_document_warn
(others?)

This means the output of "make po-check" may be incorrect as to which
files should be listed.

I won't have any more time tonight to try to fix this but will get to it
in coming days if nobody else does first.