Add the possibility to pass --flag arguments to xgettext calls

Add the possibility to pass --flag arguments to xgettext calls

The --flag argument can be used to tell xgettext the arguments of
which functions should be flagged with c-format in the PO files,
instead of guessing based on the presence of format specifiers, which
fails if no format specifiers are present but the translation
accidentally introduces one.

Appropriate flag settings have been added for each message catalog.

based on a patch by Christoph Berg for bug #6066