avoid mixture of symlinks and hardlinks, which causes problems with BSD tar on macOS Sierra;

--- a/Admin/components/components.sha1	Fri May 19 19:12:26 2017 +0200
+++ b/Admin/components/components.sha1	Fri May 19 19:41:28 2017 +0200
@@ -71,7 +71,7 @@
 741de6a4a805a0f9fb917d1845409e99346c2747  jdk-8u112.tar.gz
 5442f1015a0657259be0590b04572cd933431df7  jdk-8u11.tar.gz
 ae7df8bd0c18eb40237cf54cc28933f4893b9c92  jdk-8u121.tar.gz
-865494b7497042146282acb931a37c6a81c32dd2  jdk-8u131.tar.gz
+51531a3a0c16e180ed95cb7d2bd680c2ec0aa553  jdk-8u131.tar.gz
 cfecb1383faaf027ffbabfcd77a0b6a6521e0969  jdk-8u20.tar.gz
 44ffeeae219782d40ce6822b580e608e72fd4c76  jdk-8u31.tar.gz
 4132cf52d5025bf330d53b96a5c6466fef432377  jdk-8u51.tar.gz

--- a/src/Pure/Admin/build_jdk.scala	Fri May 19 19:12:26 2017 +0200
+++ b/src/Pure/Admin/build_jdk.scala	Fri May 19 19:41:28 2017 +0200
@@ -197,7 +197,7 @@
         } {
           val path2 = dir2.resolve(main_dir.relativize(path1))
           val file2 = path2.toFile
-          if (file2.isFile && File.eq_content(file1, file2)) {
+          if (!Files.isSymbolicLink(path2) && file2.isFile && File.eq_content(file1, file2)) {
             file2.delete
             Files.createLink(path2, path1)
           }