Use __builtin_tgmath in tgmath.h with GCC 8 (bug 21660) [committed]

[Szabolcs Nagy]

> +/* There are two variant implementations of type-generic macros in
> +   this file: one for GCC 8 and later, using __builtin_tgmath and
> +   where each macro expands each of its arguments only once, and one
> +   for older GCC, using other compiler extensions but with macros
> +   expanding their arguments many times (so resulting in exponential
> +   blowup of the size of expansions when calls to such macros are
> +   nested inside arguments to such macros).  */
> +
> +#define __HAVE_BUILTIN_TGMATH __GNUC_PREREQ (8, 0)

this breaks the build on trunk toolchains that
lack __builtin_tgmath