BLD: Do not set __STDC_VERSION__ to zero during build

The __STDC_VERSION__ set to zero prevents successful build on at least
one platform - OpenIndiana.  In addiiton, zero is not a valid value for
__STDC_VERSION__ and it is unclear why the setting was added.

Closes numpy#25366.

Author: mtelka

numpy/_core/meson.build

@@ -681,7 +681,6 @@ c_args_common = [
 
 # Same as NPY_CXX_FLAGS (TODO: extend for what ccompiler_opt adds)
 cpp_args_common = c_args_common + [
-  '-D__STDC_VERSION__=0',  # for compatibility with C headers
 ]
 if cc.get_argument_syntax() != 'msvc'
   cpp_args_common += [

numpy/_core/src/common/npy_atomic.h

@@ -9,7 +9,8 @@
 
 #include "numpy/npy_common.h"
 
-#if __STDC_VERSION__ >= 201112L && !defined(__STDC_NO_ATOMICS__)
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L \
+    && !defined(__STDC_NO_ATOMICS__)
 // TODO: support C++ atomics as well if this header is ever needed in C++
     #include <stdatomic.h>
     #include <stdint.h>