add some comments

[Bruno Haible]

I'm adding comments about a non-trivial idiom in the *.m4 files:

2009-11-22  Bruno Haible  <address@hidden>

        Add comments.
        * m4/dirent_h.m4 (gl_DIRENT_H): Add comment about gl_CHECK_NEXT_HEADERS
        invocation.
        * m4/iconv_h.m4 (gl_ICONV_H): Likewise.
        * m4/spawn_h.m4 (gl_SPAWN_H): Likewise.
        * m4/wchar.m4 (gl_WCHAR_H): Likewise.

--- m4/dirent_h.m4.orig 2009-11-23 01:00:06.000000000 +0100
+++ m4/dirent_h.m4      2009-11-23 00:59:09.000000000 +0100
@@ -1,4 +1,4 @@
-# dirent_h.m4 serial 6
+# dirent_h.m4 serial 7
 dnl Copyright (C) 2008-2009 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -12,6 +12,8 @@
   dnl once only, before all statements that occur in other macros.
   AC_REQUIRE([gl_DIRENT_H_DEFAULTS])
 
+  dnl Execute this unconditionally, because DIRENT_H may be set by other
+  dnl modules, after this code is executed.
   gl_CHECK_NEXT_HEADERS([dirent.h])
 ])
 
--- m4/iconv_h.m4.orig  2009-11-23 01:00:06.000000000 +0100
+++ m4/iconv_h.m4       2009-11-23 00:59:16.000000000 +0100
@@ -1,5 +1,5 @@
-# iconv_h.m4 serial 4
-dnl Copyright (C) 2007-2008 Free Software Foundation, Inc.
+# iconv_h.m4 serial 5
+dnl Copyright (C) 2007-2009 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.
@@ -7,6 +7,9 @@
 AC_DEFUN([gl_ICONV_H],
 [
   AC_REQUIRE([gl_ICONV_H_DEFAULTS])
+
+  dnl Execute this unconditionally, because ICONV_H may be set by other
+  dnl modules, after this code is executed.
   gl_CHECK_NEXT_HEADERS([iconv.h])
 ])
 
--- m4/spawn_h.m4.orig  2009-11-23 01:00:06.000000000 +0100
+++ m4/spawn_h.m4       2009-11-23 00:59:26.000000000 +0100
@@ -1,5 +1,5 @@
-# spawn_h.m4 serial 1
-dnl Copyright (C) 2008 Free Software Foundation, Inc.
+# spawn_h.m4 serial 2
+dnl Copyright (C) 2008-2009 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.
@@ -12,6 +12,8 @@
   dnl once only, before all statements that occur in other macros.
   AC_REQUIRE([gl_SPAWN_H_DEFAULTS])
 
+  dnl Execute this unconditionally, because SPAWN_H may be set by other
+  dnl modules, after this code is executed.
   gl_CHECK_NEXT_HEADERS([spawn.h])
 
   AC_CHECK_HEADERS_ONCE([spawn.h])
--- m4/wchar.m4.orig    2009-11-23 01:00:06.000000000 +0100
+++ m4/wchar.m4 2009-11-23 00:59:44.000000000 +0100
@@ -7,7 +7,7 @@
 
 dnl Written by Eric Blake.
 
-# wchar.m4 serial 25
+# wchar.m4 serial 26
 
 AC_DEFUN([gl_WCHAR_H],
 [
@@ -45,6 +45,8 @@
     HAVE_WCHAR_H=0
   fi
   AC_SUBST([HAVE_WCHAR_H])
+  dnl Execute this unconditionally, because WCHAR_H may be set by other
+  dnl modules, after this code is executed.
   gl_CHECK_NEXT_HEADERS([wchar.h])
 ])