gh-118124: fix assert related C++ checks on Solaris/Illumos (#121974)

Fix check for static_assert() for C++ on some platforms.

Author: kulikjak

Include/pymacro.h

@@ -15,11 +15,11 @@
 // MSVC makes static_assert a keyword in C11-17, contrary to the standards.
 //
 // In C++11 and C2x, static_assert is a keyword, redefining is undefined
-// behaviour. So only define if building as C (if __STDC_VERSION__ is defined),
-// not C++, and only for C11-17.
+// behaviour. So only define if building as C, not C++ (if __cplusplus is
+// not defined), and only for C11-17.
 #if !defined(static_assert) && (defined(__GNUC__) || defined(__clang__)) \
-     && defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L \
-     && __STDC_VERSION__ <= 201710L
+     && !defined(__cplusplus) && defined(__STDC_VERSION__) \
+     && __STDC_VERSION__ >= 201112L && __STDC_VERSION__ <= 201710L
 #  define static_assert _Static_assert
 #endif
 
@@ -47,7 +47,7 @@
 #define Py_CHARMASK(c) ((unsigned char)((c) & 0xff))
 
 #if (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L \
-     && !defined(_MSC_VER))
+     && !defined(__cplusplus) && !defined(_MSC_VER))
 #  define Py_BUILD_ASSERT_EXPR(cond) \
     ((void)sizeof(struct { int dummy; _Static_assert(cond, #cond); }), \
      0)