Fix clang -Wunused-function warnings.

Fix clang -Wunused-function warnings.

* gettext-runtime/src/envsubst.c: Include attribute.h.
(string_list_member, string_list_destroy): Mark as MAYBE_UNUSED.
* gettext-tools/src/format-scheme.c: Include attribute.h.
(II): Mark as MAYBE_UNUSED.
* gettext-tools/src/msgl-fsearch.c: Include attribute.h.
(add_index): Mark as MAYBE_UNUSED.
* gettext-tools/src/po-lex.c: Include attribute.h.
(mb_isnul, mb_cmp, mb_equal, mb_isascii, mb_putc, mb_setascii): Mark as
MAYBE_UNUSED.
* gettext-tools/src/write-qt.c: Include attribute.h.
(write_u16): Mark as MAYBE_UNUSED.
* gettext-tools/src/x-awk.c (free_token): Mark as MAYBE_UNUSED.
* gettext-tools/src/x-python.c (phase0_ungetc): Mark as MAYBE_UNUSED.