on targets where long double has the right semantics the
public math header may typedef _Float128 even if the
compiler has its own _Float128 definition.

use

#if !defined __FLT128_MANT_DIG__ || defined __cplusplus

instead of

#if !__GNUC_PREREQ (7, 0) || defined __cplusplus

to check if compiler lacks _Float128 support since it
works for both gcc and clang.

clang trunk supports _Float128 since

https://reviews.llvm.org/D40673

but they don't seem to support the f128 constant suffix
(only q) nor the __builtin_inff128 etc builtins so a bit
more change is required to make the headers work with it,
and it's not clear how to check for those features
(other than ifdef __clang__), however changing the headers
to be able to work with other compilers is useful anyway.