ilogb: Document a Mac OS X bug

[Bruno Haible]

A test failure on Mac OS X 10.5.8 (64-bit) no longer occurs since 2021-01-06.


2023-10-03  Bruno Haible  <bruno@clisp.org>

        ilogb: Document a Mac OS X bug.
        * doc/posix-functions/ilogb.texi: Mention a bug that affects Mac OS X.
        * m4/ilogb.m4 (gl_FUNC_ILOGB): Update comments.

diff --git a/doc/posix-functions/ilogb.texi b/doc/posix-functions/ilogb.texi
index edcba7c6c8..880898dd39 100644
--- a/doc/posix-functions/ilogb.texi
+++ b/doc/posix-functions/ilogb.texi
@@ -16,7 +16,7 @@
 OpenBSD 6.7, AIX 5.1.
 @item
 This function returns a wrong result for denormalized arguments on some 
platforms:
-AIX 7.1 64-bit.
+Mac OS X 10.5 64-bit, AIX 7.1 64-bit.
 @item
 This function returns a wrong result for an infinite argument on some 
platforms:
 NetBSD 7.1, OpenBSD 6.7.
diff --git a/m4/ilogb.m4 b/m4/ilogb.m4
index 257272bda7..9258c7238c 100644
--- a/m4/ilogb.m4
+++ b/m4/ilogb.m4
@@ -1,4 +1,4 @@
-# ilogb.m4 serial 8
+# ilogb.m4 serial 9
 dnl Copyright (C) 2010-2023 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -44,7 +44,8 @@ AC_DEFUN([gl_FUNC_ILOGB]
 
 dnl Test whether ilogb() works.
 dnl On OpenBSD 6.7, AIX 5.1, ilogb(0.0) is wrong.
-dnl On AIX 7.1 in 64-bit mode, ilogb(2^(DBL_MIN_EXP-1)) is wrong.
+dnl On Mac OS X 10.5 in 64-bit mode and on AIX 7.1 in 64-bit mode,
+dnl ilogb(2^(DBL_MIN_EXP-1)) is wrong.
 dnl On NetBSD 7.1, OpenBSD 6.7, ilogb(Infinity) is wrong.
 dnl On NetBSD 7.1, OpenBSD 6.7, ilogb(NaN) is wrong.
 AC_DEFUN([gl_FUNC_ILOGB_WORKS],
@@ -95,7 +96,8 @@ AC_DEFUN([gl_FUNC_ILOGB_WORKS]
     if (my_ilogb (x) != FP_ILOGB0)
       result |= 1;
   }
-  /* This test fails on AIX 7.1 in 64-bit mode.  */
+  /* This test fails on Mac OS X 10.5 in 64-bit mode and on
+     AIX 7.1 in 64-bit mode.  */
   {
     int i;
     x = 0.5;