On Cygwin, like on mingw, define DLL_EXPORT.

ChangeLog

@@ -1,3 +1,8 @@
+2006-05-06  Charles Wilson  <cygwin@cwilson.fastmail.fm>
+
+	* m4/libtool.m4: On Cygwin, like on mingw, define DLL_EXPORT when
+	compiling a shared library object.
+
 2006-05-05  Bruno Haible  <bruno@clisp.org>
 
 	* m4/libtool.m4 (AC_LIBTOOL_LANG_CXX_CONFIG): Add support for Sun C++

m4/libtool.m4

@@ -4857,12 +4857,14 @@ AC_MSG_CHECKING([for $compiler option to produce PIC])
       # like `-m68040'.
       _LT_AC_TAGVAR(lt_prog_compiler_pic, $1)='-m68020 -resident32 -malways-restore-a4'
       ;;
-    beos* | cygwin* | irix5* | irix6* | nonstopux* | osf3* | osf4* | osf5*)
+    beos* | irix5* | irix6* | nonstopux* | osf3* | osf4* | osf5*)
       # PIC is the default for these OSes.
       ;;
-    mingw* | os2* | pw32*)
+    mingw* | cygwin* | os2* | pw32*)
       # This hack is so that the source file can tell whether it is being
       # built for inclusion in a dll (and should export symbols for example).
+      # Although the cygwin gcc ignores -fPIC, still need this for old-style
+      # (--disable-auto-import) libraries
       _LT_AC_TAGVAR(lt_prog_compiler_pic, $1)='-DDLL_EXPORT'
       ;;
     darwin* | rhapsody*)
@@ -5136,13 +5138,15 @@ AC_MSG_CHECKING([for $compiler option to produce PIC])
       _LT_AC_TAGVAR(lt_prog_compiler_pic, $1)='-m68020 -resident32 -malways-restore-a4'
       ;;
 
-    beos* | cygwin* | irix5* | irix6* | nonstopux* | osf3* | osf4* | osf5*)
+    beos* | irix5* | irix6* | nonstopux* | osf3* | osf4* | osf5*)
       # PIC is the default for these OSes.
       ;;
 
-    mingw* | pw32* | os2*)
+    mingw* | cygwin* | pw32* | os2*)
       # This hack is so that the source file can tell whether it is being
       # built for inclusion in a dll (and should export symbols for example).
+      # Although the cygwin gcc ignores -fPIC, still need this for old-style
+      # (--disable-auto-import) libraries
       _LT_AC_TAGVAR(lt_prog_compiler_pic, $1)='-DDLL_EXPORT'
       ;;
 
@@ -5210,9 +5214,11 @@ AC_MSG_CHECKING([for $compiler option to produce PIC])
        esac
        ;;
 
-    mingw* | pw32* | os2*)
+    mingw* | cygwin* | pw32* | os2*)
       # This hack is so that the source file can tell whether it is being
       # built for inclusion in a dll (and should export symbols for example).
+      # Although the cygwin gcc ignores -fPIC, still need this for old-style
+      # (--disable-auto-import) libraries
       _LT_AC_TAGVAR(lt_prog_compiler_pic, $1)='-DDLL_EXPORT'
       ;;