This is the last entity in config/powerpc/xm-aix4.h. The comment says:
/* UINT_MAX is defined in <sys/limits.h> as a decimal constant (4294967295)
which is too large to fit in a signed int when it is parsed by the
compiler, so it issues a diagnostic. Just undef it here so that we
use gdb's version in defs.h */
#undef UINT_MAX
So we have a collision between the macro defined by GDB and the macro
defined by the system. For recent enough versions of AIX, I don't think
the values are different, so we could look the other way and just remove
the #undef, and all things should work. But we still have this collision
and I think we should fix it.