[3.12] gh-118124: fix assert related C++ checks on Solaris/Illumos (GH-121974) (#122109)

Fix check for static_assert() for C++ on some platforms..
(cherry picked from commit e88bd96)

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