Auto merge of #1319 - jasonbking:readdir, r=gnzlbg

This fixes issue #1318 - on Solarish systems __posix_readdir_r is only

defined for 32-bit Solarish targets.  Since rust doesn't currently support 32-bit Solarish targets, the line is both unnecessary and wrong.

As found with the latest fd binary, removing the line fixed the fd build (and fd then works as expected).

src/unix/mod.rs

@@ -612,9 +612,13 @@ extern {
     pub fn readdir(dirp: *mut ::DIR) -> *mut ::dirent;
     #[cfg_attr(target_os = "macos", link_name = "readdir_r$INODE64")]
     #[cfg_attr(target_os = "netbsd", link_name = "__readdir_r30")]
-    #[cfg_attr(any(target_os = "solaris", target_os = "illumos"),
-               link_name = "__posix_readdir_r")]
     #[cfg_attr(target_os = "freebsd", link_name = "readdir_r@FBSD_1.0")]
+    /// The 64-bit libc on Solaris and illumos only has readdir_r.  If a
+    /// 32-bit Solaris or illumos target is ever created, it should use
+    /// __posix_readdir_r.  See libc(3LIB) on Solaris or illumos:
+    /// https://illumos.org/man/3lib/libc
+    /// https://docs.oracle.com/cd/E36784_01/html/E36873/libc-3lib.html
+    /// https://www.unix.com/man-page/opensolaris/3LIB/libc/
     pub fn readdir_r(dirp: *mut ::DIR, entry: *mut ::dirent,
                      result: *mut *mut ::dirent) -> ::c_int;
     #[cfg_attr(all(target_os = "macos", target_arch = "x86"),