canonicalize on mingw

[Bruno Haible]

mingw doesn't have symlinks, therefore S_ISLNK (defined in stat-macros.h)
always returns 0. This should avoid the compilation error. canonicalize.m4
should then also check for readlink().

*** canonicalize.c.bak  2006-09-19 00:51:16.000000000 +0200
--- canonicalize.c      2006-10-20 03:37:12.000000000 +0200
***************
*** 230,235 ****
--- 230,236 ----
              st.st_mode = 0;
            }
  
+ #if HAVE_READLINK
          if (S_ISLNK (st.st_mode))
            {
              char *buf;
***************
*** 282,287 ****
--- 283,289 ----
              free (buf);
            }
          else
+ #endif
            {
              if (!S_ISDIR (st.st_mode) && *end && (can_mode != CAN_MISSING))
                {