libgettextpo: Remove static variable 'result.0'.
* gettext-tools/src/write-po.h (make_format_description_string): Change return
type to 'char *'.
* gettext-tools/src/write-po.c (make_format_description_string): Return a
freshly allocated string.
(message_print_comment_flags, message_print_obsolete): Free the result of
make_format_description_string when done.
* gettext-tools/src/write-stringtable.c (write_message): Likewise.
