Problem reported by Harald Maier (Bug#26398).
The macOS C compiler uses __nonnull for its own purposes and that
clashes with glibc's __nonnull.
* lib/getopt.in.h: Add comment for _GL_ARG_NONNULL snippet.
* lib/getopt_cdefs.in.h (__nonnull): Remove.
* lib/getopt_core.h (getopt):
* lib/getopt_ext.h (getopt_long, getopt_long_only):
Use _GL_ARG_NONNULL, not __nonnull.
* lib/unistd.in.h: Move snippet hooks to before where the getopt
.h files are included, so that _GL_ARG_NONNULL is defined in time.
* modules/getopt-posix (Depends-on): Add snippet/arg-nonnull.
(getopt.h): Interpolate _GL_ARG_NONNULL snippet.