C++0x patch for boost::unit

[Chris Jefferson]

The attached patch makes boost::unit work correctly in C++0x, by making the code NOT compile when a dimension ordinal is used more than once.

At the moment this code is only needed in clang's c++0x mode, but it doesn't seem to break g++ c++03 or c++0x, so put it all the time.

The technique currently used (friend functions being double declared) doesn't work any more in C++0x, as:

C++03 says: When a function is defined in a friend function declaration in a class template, the function is defined at each instantiation of the class template. The function is defined even if it is never used.

C++0x says: When a function is defined in a friend function declaration in a class template, the function is instantiated when the function is odr-used.

The new technique makes functions with different return values, which still causes an error.

[Steven Watanabe]

(In [71081]) Fix checking for double registration with clang in C++0x mode. Fixes #5436.