re-enable fresh compilation on cygwin

Ever since late January, using CVS libtool made it impossible to link on cygwin 
(see the thread in [1] for details; since I often develop on cygwin, I have 
just been avoiding a fresh bootstrap for nearly a month now).  This patch picks 
up the recommendation in [2] for working around the issue, without having to 
explicitly decorate all symbols in all the libraries.

[1] http://lists.gnu.org/archive/html/libtool-patches/2007-01/msg00041.html
[2] http://lists.gnu.org/archive/html/libtool-patches/2007-02/msg00062.html

2007-02-26  Eric Blake  <[email protected]>

	Re-enable clean bootstrap on cygwin.
	* Makefile.am (AM_LDFLAGS): For now, export all symbols from
	modules, since otherwise we run afoul of current libtool
	limitations on cygwin and mingw.

Index: Makefile.am
===================================================================
RCS file: /sources/m4/m4/Makefile.am,v
retrieving revision 1.51
diff -u -r1.51 Makefile.am
--- Makefile.am	27 Oct 2006 12:12:12 -0000	1.51
+++ Makefile.am	26 Feb 2007 21:29:35 -0000
@@ -1,6 +1,6 @@
 ## This file is part of GNU M4.
 ##
-## Copyright (C) 2000, 2001, 2003, 2004, 2005, 2006 Free Software
+## Copyright (C) 2000, 2001, 2003, 2004, 2005, 2006, 2007 Free Software
 ## Foundation, Inc.
 ##
 ## This program is free software; you can redistribute it and/or modify
@@ -31,7 +31,12 @@
 		  -Ignu -I$(srcdir)/gnu \
 		  -Im4 -I$(srcdir)/m4 \
 		  $(LTDLINCL)
-AM_LDFLAGS	= -no-undefined -export-dynamic
+# FIXME - the -export-symbols-regex ".*" is a hack to work around the
+# fact that on cygwin, the convenience library libltdl exports symbols,
+# which effectively disables the auto-exporting of all other symbols.
+# Revisit this if libtool on cygwin improves.
+# See http://lists.gnu.org/archive/html/libtool-patches/2007-02/msg00062.html
+AM_LDFLAGS	= -no-undefined -export-dynamic -export-symbols-regex ".*"
 
 EXTRA_DIST	= bootstrap \
 		  $(config_aux_dir)/mkstamp \