Harald Maier wrote:
> They __THROW and __nonnull are defined in „getopt_cdefs.h“.

Thanks, I think I see the problem. Either the compiler or some macOS header does 
"#define __nonnull _Nonnull", and this collides with glibc's use of __nonnull. 
Presumably Clang uses _Nonnull as a type qualifier and some Clang headers use 
__nonnull as an alias for _Nonnull.

Please try the attached patch against Emacs master. If it works I plan to 
install it into Emacs and propagate it into Gnulib.