new file from Eric Blake

m4/double-slash-root.m4

@@ -0,0 +1,43 @@
+#serial 1   -*- autoconf -*-
+dnl Copyright (C) 2006 Free Software Foundation, Inc.
+dnl This file is free software; the Free Software Foundation
+dnl gives unlimited permission to copy and/or distribute it,
+dnl with or without modifications, as long as this notice is preserved.
+
+AC_DEFUN([gl_DOUBLE_SLASH_ROOT],
+[
+  AC_REQUIRE([AC_CANONICAL_HOST])
+  AC_CACHE_CHECK([whether // is distinct from /], [ac_cv_double_slash_root],
+    [ if test x"$cross_compiling" = xyes ; then
+	# When cross-compiling, there is no way to tell whether // is special
+	# short of a list of hosts.  However, the only known hosts to date
+	# that have a distinct // are Apollo DomainOS (too old to port to)
+	# and Cygwin.  If anyone knows of another system for which // has
+	# special semantics and is distinct from /, please report it to
+	# <bug-coreutils@gnu.org>.
+	case $host in
+	  *-cygwin)
+	    ac_cv_double_slash_root=yes ;;
+	  *)
+	    # Be optimistic and assume that / and // are the same when we
+	    # don't know.
+	    ac_cv_double_slash_root='unknown, assuming no' ;;
+	esac
+      else
+	set x `ls -di / //`
+	if test $[2] = $[4]; then
+	  ac_cv_double_slash_root=no
+	else
+	  ac_cv_double_slash_root=yes
+	fi
+      fi])
+  if test x"$ac_cv_double_slash_root" = xyes; then
+    ac_double_slash_root=1
+  else
+    ac_double_slash_root=0
+  fi
+
+  AC_DEFINE_UNQUOTED([DOUBLE_SLASH_IS_DISTINCT_ROOT],
+   $ac_double_slash_root,
+   [Define to 1 if // is a file system root distinct from /.])
+])