Fix "warning: cast from 'const char *' to 'char *' drops const qualifier".
* gettext-tools/src/str-list.h (string_list_append_unique_desc,
string_list_member_desc): New declarations.
* gettext-tools/src/str-list.c (string_list_append_unique_desc,
string_list_member_desc): New functions.
* gettext-tools/src/msgfmt.c (add_languages): Don't modify the contents of
'line'.
