_Noreturn.h not respecting __STDC_VERSION__ (macOS, clang 10)

[Joe Nelson]

Hi, my project uses lib/_Noreturn.h, but gets a warning when compiled
with "-std=c99 -pedantic":

        warning: _Noreturn functions are a C11-specific feature

By placing an #error directive in the code below during testing, I
confirmed the preprocessor is incorrectly hitting the condition marked
/* _Noreturn works as-is.  */

        #ifndef _Noreturn
        # if (defined __cplusplus \
                  && ((201103 <= __cplusplus && !(__GNUC__ == 4 && 
__GNUC_MINOR__ == 7)) \
                          || (defined _MSC_VER && 1900 <= _MSC_VER)) \
                  && 0)
                /* [[noreturn]] is not practically usable, because with it the 
syntax
                         extern _Noreturn void func (...);
                   would not be valid; such a declaration would only be valid 
with 'extern'
                   and '_Noreturn' swapped, or without the 'extern' keyword.  
However, some
                   AIX system header files and several gnulib header files use 
precisely
                   this syntax with 'extern'.  */
        #  define _Noreturn [[noreturn]]
        # elif ((!defined __cplusplus || defined __clang__)                     
\
                        && (201112 <= (defined __STDC_VERSION__ ? 
__STDC_VERSION__ : 0)  \
                                || 4 < __GNUC__ + (7 <= __GNUC_MINOR__) \
                                || (defined __apple_build_version__ \
                                        ? 6000000 <= __apple_build_version__ \
                                        : 3 < __clang_major__ + (5 <= 
__clang_minor__))))
           /* _Noreturn works as-is.  */
        # elif 2 < __GNUC__ + (8 <= __GNUC_MINOR__) || 0x5110 <= __SUNPRO_C
        #  define _Noreturn __attribute__ ((__noreturn__))
        # elif 1200 <= (defined _MSC_VER ? _MSC_VER : 0)
        #  define _Noreturn __declspec (noreturn)
        # else
        #  define _Noreturn
        # endif
        #endif

The correct behavior would be to respect the value of __STDC_VERSION__
which I set with the -std=c99 compiler flag.

Here are the values of the relevant macros in my environment:

#define __GNUC__ 4
#define __GNUC_MINOR__ 2
#define __STDC_VERSION__ 199901L
#define __apple_build_version__ 10001145
#define __clang_major__ 10
#define __clang_minor__ 0

Here is my compiler version:
        Apple LLVM version 10.0.0 (clang-1000.11.45.5)
        Target: x86_64-apple-darwin17.7.0

Also tested that on my system it would be OK to define _Noreturn to be
"__attribute__ ((__noreturn__))".  That option causes no warnings.

Thanks for your help. Hope there's a way to simplify/fix the logic in
the header.