Don't redefine _Py_NULL macro if already defined (#138)

Original mypy fix by Michael R. Crusoe.

Author: vstinner

pythoncapi_compat.h

@@ -37,11 +37,13 @@ extern "C" {
 // Static inline functions should use _Py_NULL rather than using directly NULL
 // to prevent C++ compiler warnings. On C23 and newer and on C++11 and newer,
 // _Py_NULL is defined as nullptr.
-#if (defined (__STDC_VERSION__) && __STDC_VERSION__ > 201710L) \
-        || (defined(__cplusplus) && __cplusplus >= 201103)
-#  define _Py_NULL nullptr
-#else
-#  define _Py_NULL NULL
+#ifndef _Py_NULL
+#  if (defined (__STDC_VERSION__) && __STDC_VERSION__ > 201710L) \
+          || (defined(__cplusplus) && __cplusplus >= 201103)
+#    define _Py_NULL nullptr
+#  else
+#    define _Py_NULL NULL
+#  endif
 #endif
 
 // Cast argument to PyObject* type.