Require symlink support

Require lstat(2), lchown(2), S_IFLNK and S_ISLNK from POSIX.1-2001.

Already unconditionally used in lib/tcbfuncs.c and lib/run_part.c.

lib/commonio.c

@@ -65,7 +65,6 @@ int lrename (const char *old, const char *new)
 	int res;
 	char *r = NULL;
 
-#if defined(S_ISLNK)
 #ifndef __GLIBC__
 	char resolved_path[PATH_MAX];
 #endif				/* !__GLIBC__ */
@@ -82,7 +81,6 @@ int lrename (const char *old, const char *new)
 			new = r;
 		}
 	}
-#endif				/* S_ISLNK */
 
 	res = rename (old, new);