Use -fvisibility=hidden when it's available.

configure.ac

@@ -72,6 +72,12 @@ AC_CHECK_LIB(ws2_32, getsockname)
 AC_CHECK_LIB(shlwapi, PathRemoveFileSpecA)
 AC_CHECK_FUNCS([backtrace_symbols execvpe pipe2])
 
+AX_C___ATTRIBUTE__
+if test "$ax_cv___attribute__" = "yes"; then
+	CFLAGS="$CFLAGS -fvisibility=hidden"
+	CXXFLAGS="$CXXFLAGS -fvisibility=hidden"
+fi
+
 AC_MSG_CHECKING(whether to enable debugging)
 AC_ARG_ENABLE(debug, [  --enable-debug=[no/yes]   turn on debugging (default=no)],, enable_debug=no)
 if test "x$enable_debug" = "xyes"; then

lib/base/unix.h

@@ -49,8 +49,12 @@ typedef int SOCKET;
 #	define MAXPATHLEN PATH_MAX
 #endif /* MAXPATHLEN */
 
-/* default visibility takes care of exported symbols */
+#if HAVE___ATTRIBUTE__
-#define I2_EXPORT
+#	define I2_EXPORT __attribute__ ((visibility ("default")))
-#define I2_IMPORT
+#	define I2_IMPORT __attribute__ ((visibility ("default")))
-
+#else /* HAVE___ATTRIBUTE__ */
+/* default visibility takes care of exported symbols if we don't have __attribute__ */
+#	define I2_EXPORT
+#	define I2_IMPORT
+#endif /* HAVE___ATTRIBUTE__ */
 #endif /* UNIX_H */

m4/ax_c___attribute__.m4

@@ -0,0 +1,66 @@
+# ===========================================================================
+#    http://www.gnu.org/software/autoconf-archive/ax_c___attribute__.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AX_C___ATTRIBUTE__
+#
+# DESCRIPTION
+#
+#   Provides a test for the compiler support of __attribute__ extensions.
+#   Defines HAVE___ATTRIBUTE__ if it is found.
+#
+# LICENSE
+#
+#   Copyright (c) 2008 Stepan Kasal <skasal@redhat.com>
+#   Copyright (c) 2008 Christian Haggstrom
+#   Copyright (c) 2008 Ryan McCabe <ryan@numb.org>
+#
+#   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 of the License, 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, see <http://www.gnu.org/licenses/>.
+#
+#   As a special exception, the respective Autoconf Macro's copyright owner
+#   gives unlimited permission to copy, distribute and modify the configure
+#   scripts that are the output of Autoconf when processing the Macro. You
+#   need not follow the terms of the GNU General Public License when using
+#   or distributing such scripts, even though portions of the text of the
+#   Macro appear in them. The GNU General Public License (GPL) does govern
+#   all other use of the material that constitutes the Autoconf Macro.
+#
+#   This special exception to the GPL applies to versions of the Autoconf
+#   Macro released by the Autoconf Archive. When you make and distribute a
+#   modified version of the Autoconf Macro, you may extend this special
+#   exception to the GPL to apply to your modified version as well.
+
+#serial 8
+
+AC_DEFUN([AX_C___ATTRIBUTE__], [
+  AC_CACHE_CHECK([for __attribute__], [ax_cv___attribute__],
+    [AC_COMPILE_IFELSE(
+      [AC_LANG_PROGRAM(
+	[[#include <stdlib.h>
+	  static void foo(void) __attribute__ ((unused));
+	  static void
+	  foo(void) {
+	      exit(1);
+	  }
+        ]], [])],
+      [ax_cv___attribute__=yes],
+      [ax_cv___attribute__=no]
+    )
+  ])
+  if test "$ax_cv___attribute__" = "yes"; then
+    AC_DEFINE([HAVE___ATTRIBUTE__], 1, [define if your compiler has __attribute__])
+  fi
+])