--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,14 @@
+2011-10-09  Bruno Haible  <bruno@clisp.org>
+
+	truncl: Simplify for platforms where 'long double' == 'double'.
+	* lib/truncl.c: Include <config.h>.
+	(truncl) [HAVE_SAME_LONG_DOUBLE_AS_DOUBLE]: New function.
+	* lib/trunc.c [!USE_LONG_DOUBLE]: Don't include <config.h> a second
+	time.
+	* m4/truncl.m4 (gl_FUNC_TRUNCL): Require gl_LONG_DOUBLE_VS_DOUBLE.
+	Determine TRUNCL_LIBM according to HAVE_SAME_LONG_DOUBLE_AS_DOUBLE.
+	* modules/truncl (Depends-on): Add trunc. Update conditions.
+
 2011-10-09  Bruno Haible  <bruno@clisp.org>
 
 	ceill: Simplify for platforms where 'long double' == 'double'.

--- a/lib/trunc.c
+++ b/lib/trunc.c
@@ -16,7 +16,9 @@
 
 /* Written by Bruno Haible <bruno@clisp.org>, 2007.  */
 
-#include <config.h>
+#if ! defined USE_LONG_DOUBLE
+# include <config.h>
+#endif
 
 /* Specification.  */
 #include <math.h>

--- a/lib/truncl.c
+++ b/lib/truncl.c
@@ -16,5 +16,22 @@
 
 /* Written by Bruno Haible <bruno@clisp.org>, 2007.  */
 
-#define USE_LONG_DOUBLE
-#include "trunc.c"
+#include <config.h>
+
+#if HAVE_SAME_LONG_DOUBLE_AS_DOUBLE
+
+/* Specification.  */
+# include <math.h>
+
+long double
+truncl (long double x)
+{
+  return trunc (x);
+}
+
+#else
+
+# define USE_LONG_DOUBLE
+# include "trunc.c"
+
+#endif

--- a/m4/truncl.m4
+++ b/m4/truncl.m4
@@ -1,4 +1,4 @@
-# truncl.m4 serial 9
+# truncl.m4 serial 10
 dnl Copyright (C) 2007-2008, 2010-2011 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -8,9 +8,12 @@
 [
   m4_divert_text([DEFAULTS], [gl_truncl_required=plain])
   AC_REQUIRE([gl_MATH_H_DEFAULTS])
+  AC_REQUIRE([gl_LONG_DOUBLE_VS_DOUBLE])
   AC_REQUIRE([AC_CANONICAL_HOST]) dnl for cross-compiles
+
   dnl Persuade glibc <math.h> to declare truncl().
   AC_REQUIRE([gl_USE_SYSTEM_EXTENSIONS])
+
   dnl Test whether truncl() is declared.
   AC_CHECK_DECLS([truncl], , , [[#include <math.h>]])
   if test "$ac_cv_have_decl_truncl" = yes; then
@@ -109,8 +112,13 @@
     HAVE_DECL_TRUNCL=0
   fi
   if test $HAVE_DECL_TRUNCL = 0 || test $REPLACE_TRUNCL = 1; then
-    dnl No libraries are needed to link lib/truncl.c.
-    TRUNCL_LIBM=
+    dnl Find libraries needed to link lib/truncl.c.
+    if test $HAVE_SAME_LONG_DOUBLE_AS_DOUBLE = 1; then
+      AC_REQUIRE([gl_FUNC_TRUNC])
+      TRUNCL_LIBM="$TRUNC_LIBM"
+    else
+      TRUNCL_LIBM=
+    fi
   fi
   AC_SUBST([TRUNCL_LIBM])
 ])

--- a/modules/truncl
+++ b/modules/truncl
@@ -9,7 +9,8 @@
 Depends-on:
 math
 extensions
-float           [test $HAVE_DECL_TRUNCL = 0 || test $REPLACE_TRUNCL = 1]
+trunc           [{ test $HAVE_DECL_TRUNCL = 0 || test $REPLACE_TRUNCL = 1; } && test $HAVE_SAME_LONG_DOUBLE_AS_DOUBLE = 1]
+float           [{ test $HAVE_DECL_TRUNCL = 0 || test $REPLACE_TRUNCL = 1; } && test $HAVE_SAME_LONG_DOUBLE_AS_DOUBLE = 0]
 
 configure.ac:
 gl_FUNC_TRUNCL