[bind] inline placeholder macro

[daniel_walker]

This patch introduces and documents a macro to enable placeholders implemented as inline functions instead of static objects for compilers other than borland and g++ <= 4.0. Also, it gives g++ > 4.0 static object placeholders by default. This is important for the TR1 CopyConstructible requirement and for practical reasons when implementing operator overloads for placeholders. Finally, the patch also introduces a macro containing gcc's complete version number for convenient use with BOOST_WORKAROUND. See mailing list thread archived at http://tinyurl.com/yqvp8y.