diff --git a/unix/m4/longdouble.m4 b/unix/m4/longdouble.m4
new file mode 100644
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--- /dev/null
+++ b/unix/m4/longdouble.m4
@@ -0,0 +1,28 @@
+# longdouble.m4 serial 1 (gettext-0.12)
+dnl Copyright (C) 2002-2003 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.
+
+dnl From Bruno Haible.
+dnl Test whether the compiler supports the 'long double' type.
+dnl Prerequisite: AC_PROG_CC
+
+AC_DEFUN([gt_TYPE_LONGDOUBLE],
+[
+  AC_CACHE_CHECK([for long double], gt_cv_c_long_double,
+    [if test "$GCC" = yes; then
+       gt_cv_c_long_double=yes
+     else
+       AC_TRY_COMPILE([
+         /* The Stardent Vistra knows sizeof(long double), but does not support it.  */
+         long double foo = 0.0;
+         /* On Ultrix 4.3 cc, long double is 4 and double is 8.  */
+         int array [2*(sizeof(long double) >= sizeof(double)) - 1];
+         ], ,
+         gt_cv_c_long_double=yes, gt_cv_c_long_double=no)
+     fi])
+  if test $gt_cv_c_long_double = yes; then
+    AC_DEFINE(HAVE_LONG_DOUBLE, 1, [Define if you have the 'long double' type.])
+  fi
+])