* lib/gl_anytree_list2.h: Add _GL_ATTRIBUTE_PURE to avoid
-Werror=suggest-attribute=pure.
* lib/gl_array_list.c: Likewise.
* lib/gl_avltree_list.c (gl_avltree_list_check_invariants): Add extern
declaration to avoid -Werror=missing-prototypes. This is not added
to a header as only exported for tests. Add (void) to the
check_invariants() call to indicate we're discarding the result
in this context which avoids -Werror=unused-value. Note we don't
use ignore_value here to avoid a dependency as we know we'll not
be adding __attribute__((warn_unused_result)) to check_invariants().
Add _GL_ATTRIBUTE_CONST to avoid -Werror=suggest-attribute=const.