Macros for exporting symbols on Cygwin.

diff --git a/gettext-tools/cygwin/export.h b/gettext-tools/cygwin/export.h
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+/* Exporting symbols from Cygwin shared libraries.
+   Copyright (C) 2006 Free Software Foundation, Inc.
+   Written by Bruno Haible <bruno@clisp.org>, 2006.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 2, or (at your option)
+   any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, write to the Free Software Foundation,
+   Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */
+
+/* There are four ways to build shared libraries on Cygwin:
+
+   - Export only functions, no variables.
+     This has the drawback of severely affecting the programming style in use.
+     It does not let the programmer use full ANSI C.  It lets one platform
+     dictate the code style on all platforms.  This is unacceptable.
+
+   - Use the GNU ld --enable-auto-import option.  It is the default on Cygwin
+     since July 2005. But it three fatal drawbacks:
+       - It produces executables and shared libraries with relocations in the
+         .text segment, defeating the principles of virtual memory.
+       - For some constructs such as
+             extern int var;
+             int * const b = &var;
+         it creates an executable that will give an error at runtime, rather
+         than either a compile-time or link-time error or a working executable.
+         (This is with both gcc and g++.) Whereas this code, not relying on
+         auto-import:
+             extern __declspec (dllimport) int var;
+             int * const b = &var;
+         gives a compile-time error with gcc and works with g++.
+       - It doesn't work in some cases (references to a member field of an
+         exported struct variable, or to a particular element of an exported
+         array variable), requiring code modifications.  Again one platform
+         dictates code modifications on all platforms.
+
+     This is unacceptable.  Therefore we disable this option, through the
+     woe32-dll.m4 autoconf macro.
+
+   - Define a macro that expands to  __declspec(dllexport)  when building
+     the library and to  __declspec(dllimport)  when building code outside
+     the library, and use it in all header files of the library.
+     This is acceptable if
+       1. the header files are unique to this library (not shared with
+          other packages), and
+       2. the library sources are contained in one directory, making it easy
+          to define a -DBUILDING_LIBXYZ flag for the library.
+     Example:
+         #ifdef BUILDING_LIBASPRINTF
+         #define LIBASPRINTF_DLL_EXPORTED __declspec(dllexport)
+         #else
+         #define LIBASPRINTF_DLL_EXPORTED __declspec(dllimport)
+         #endif
+
+     We use this technique for the libintl and the libasprintf libraries.
+
+   - Define a macro that expands to  __declspec(dllimport)  always, and use
+     it in all header files of the library.  Use an explicit export list for
+     the library.
+     This is acceptable if
+       1. the programming language is not C++ (because the name mangling of
+          static struct/class fields and of variables in namespaces makes it
+          hard to maintain an export list).
+     The benefit of this approach is that the partitioning of the source files
+     into libraries (which source file goes into which library) does not
+     affect the source code; only the Makefiles reflect it.
+     The performance loss due to the unnecessary indirection for references
+     to variables from within the library defining the variable is acceptable.
+
+     We use this technique for libgettextlib (because it contains many gnulib
+     modules) and for libgettextsrc (because this makes it easy to move source
+     code from an msg* program to libgettextsrc).  The macro is called
+     DLL_VARIABLE.
+
+   This file allows building an explicit export list.  You can either
+     - specify the variables to be exported, and use the GNU ld option
+       --export-all-symbols to export all function names, or
+     - specify the variables and functions to be exported explicitly.
+
+   Note: --export-all-symbols is the default when no other symbol is explicitly
+   exported.  This means, the use of an explicit export on the variables has
+   the effect of no longer exporting the functions! - until the option
+   --export-all-symbols is used.  */
+
+/* libtool defines the symbol DLL_EXPORT when compiling the object files that
+   will go into a shared library.  */
+#ifdef DLL_EXPORT
+ /* Compiling for a shared library.  */
+
+ /* IMP(x) is a symbol that contains the address of x.  */
+# define IMP(x) _imp__##x
+
+ /* Ensure that the variable x is exported from the library, and that a
+    pseudo-variable IMP(x) is available.  */
+# define VARIABLE(x) \
+  /* Export x without redefining x.  This code was found by compiling a        \
+     snippet:                                                          \
+       extern __declspec(dllexport) int x; int x = 42;  */             \
+  asm (".section .drectve\n");                                         \
+  asm (".ascii \" -export:" #x ",data\"\n");                           \
+  asm (".data\n");                                                     \
+  /* Allocate a pseudo-variable IMP(x).  */                            \
+  extern int x;                                                                \
+  void * IMP(x) = &x;
+
+ /* Ensure that the function x is exported from the library.  */
+# define FUNCTION(x) \
+  /* Export x without redefining x.  This code was found by compiling a        \
+     snippet:                                                          \
+       extern __declspec(dllexport) void x (void);  */                 \
+  asm (".section .drectve\n");                                         \
+  asm (".ascii \" -export:" #x "\"\n");                                        \
+  asm (".data\n");
+
+#else
+  /* Compiling for a static library.  */
+
+# define VARIABLE(x)
+# define FUNCTION(x)
+
+#endif