Support separate function declaration and definition with ENABLE_IF-based SFINAE

The ENABLE_IF macro provides convenient syntax for function template SFINAE using the new default template parameter feature of C++11. However, this macro can only be used in the function declaration (which may also be a definition).

The syntax needed in a definition that is separate from the declaration is different, but the type forms for the non-type template parameters in the two places must be "equivalent" (C++14 14.5.6). The precise form for ENABLE_IF is "hidden" behind the macro. It is not desirable to have template definitions making assumptions about that form.

This suggests there should be a second macro for use in the separate definition case, with the two macros maintained together to ensure the necessary consistency.