[boost] [mpl] mpl::if_ workaround for digital mars

Aleksey, I've just been trying the new version of mpl on Digital Mars. The attached patch fixes the bug that sometimes causes mpl::if_ to fail. The problem is that the class template gets redeclared with different parameter names by bind.hpp, and this confuses the compiler. When it instantiates the class, it uses the wrong names. The fix is just to make sure bind.hpp is included before mpl::if_ is defined. thanks, Daniel