[Bug-readline] colors.c does not compile under some mingw versions

[Christian Biesinger]

Hello!

In compiling Readline 8.0 under mingw (Windows), some versions do not
compile because of this line in colors.c:

  else if (linkok == 0 && S_ISLNK(mode) &&
_rl_color_indicator[C_ORPHAN].string != NULL)

They do not support S_ISLNK. All other usages of S_ISLNK are wrapped
in either an #ifdef HAVE_LSTAT or #ifdef S_ISLNK.

So it seems that this usae should also have such an ifdef. However, an
even easier fix would be to just remove S_ISLNK from this condition,
because linkok can only be 0 if S_ISLNK(mode) is true, based on the
code above.

Hopefully a fix can be made accordingly to readline!

Thanks,
Christian