[Patch] [libstdc++] [constexpr] use macros in place of 'constexpr' to allow for disabling constexpr until fully supported by compiler

[Gabriel Dos Reis]

I certainly agree that we should resist the __GBLICXX_CONSTEXPR macro.
The current semantics implemented by the compiler is not going to meet
anybody's expectation by the time GCC is released.  I don't see the point
of introducing the macro.

-- Gaby