insist in proper GNU tar, to avoid subtle semantic differences;

--- a/src/Pure/Admin/build_jdk.scala	Sun Jan 22 15:02:06 2017 +0100
+++ b/src/Pure/Admin/build_jdk.scala	Sun Jan 22 16:15:08 2017 +0100
@@ -106,8 +106,8 @@
     try {
       val tmp_dir = dir + Path.explode("tmp")
       Isabelle_System.mkdirs(tmp_dir)
-      Isabelle_System.bash(
-        "tar -C " + File.bash_path(tmp_dir) + " -xzf " + File.bash_path(archive)).check
+      Isabelle_System.gnutar(
+        "-C " + File.bash_path(tmp_dir) + " -xzf " + File.bash_path(archive)).check
       val dir_entry =
         File.read_dir(tmp_dir) match {
           case List(s) => s
@@ -204,7 +204,7 @@
         }
 
         progress.echo("Archiving ...")
-        Isabelle_System.bash("tar -C " + File.bash_path(dir) + " -czf " +
+        Isabelle_System.gnutar("-C " + File.bash_path(dir) + " -czf " +
           File.bash_path(target_dir + jdk_path.ext("tar.gz")) + " " + jdk_name).check
       })
   }

--- a/src/Pure/Admin/build_release.scala	Sun Jan 22 15:02:06 2017 +0100
+++ b/src/Pure/Admin/build_release.scala	Sun Jan 22 16:15:08 2017 +0100
@@ -155,11 +155,13 @@
           {
             def execute(script: String): Unit =
               Isabelle_System.bash(script, cwd = tmp_dir.file).check
+            def execute_tar(args: String): Unit =
+              Isabelle_System.gnutar(args, cwd = tmp_dir.file).check
 
             val name = release_info.name
             val platform = Isabelle_System.getenv_strict("ISABELLE_PLATFORM_FAMILY")
             val bundle = release_info.dist_dir + Path.explode(name + "_" + platform + ".tar.gz")
-            execute("tar xzf " + File.bash_path(bundle))
+            execute_tar("xzf " + File.bash_path(bundle))
 
             val other_isabelle =
               new Other_Isabelle(progress, tmp_dir + Path.explode(name), name + "-build")
@@ -171,7 +173,7 @@
 
             execute("chmod -R a+r " + Bash.string(name))
             execute("chmod -R g=o " + Bash.string(name))
-            execute("tar czf " + File.bash_path(release_info.dist_library_archive) +
+            execute_tar("czf " + File.bash_path(release_info.dist_library_archive) +
               " " + Bash.string(name + "/browser_info"))
           })
       }

--- a/src/Pure/System/isabelle_system.scala	Sun Jan 22 15:02:06 2017 +0100
+++ b/src/Pure/System/isabelle_system.scala	Sun Jan 22 16:15:08 2017 +0100
@@ -302,6 +302,16 @@
       result(progress_stdout, progress_stderr, progress_limit, strict)
   }
 
+  private lazy val gnutar_check: Boolean =
+    try { bash("tar --version").check.out.containsSlice("GNU tar") || error("") }
+    catch { case ERROR(_) => false }
+
+  def gnutar(args: String, cwd: JFile = null, redirect: Boolean = false): Process_Result =
+  {
+    if (gnutar_check) bash("tar " + args, cwd = cwd, redirect = redirect)
+    else error("Expected to find GNU tar executable")
+  }
+
   def hostname(): String = bash("hostname -s").check.out
 
   def open(arg: String): Unit =