In the most recent GSString.m version these methods are there only
once.
The first one is obsolete anyway, see NSString, where it gets defined
as
calling the second one. As this in now the only remaining usage of
substring_c, we could try to do the inlining ourselves and put the code
from the function inside the method. If this alone does not resolve the
compilation issue, we could try to rework the actual code. Here the
interesting question could be why the similar which calls substring_u
works.