For C++ extensions, use the pthreads definition of PERL_GET_CONTEXT

Configure probes the C compiler to find out whether it supports C11 thread
local storage, and if found uses this for PERL_SET_CONTEXT/PERL_GET_CONTEXT,
in preference to the pthread_setspecific()/pthread_getspecific() approach.

However, we can come unstuck with XS extensions written in C++, as on some
platforms the C++ compiler can use different syntax for thread local storage.
As Configure doesn't have a way to probe for C++ dialects, we just take the
safe option and do the same as 5.34.0 and earlier - use pthreads on C++.

This commit is minimal because the implementation of PERL_SET_CONTEXT for
C11 thread local storage was already defensively written to *also* call
pthread_setspecific().

thread.h

@@ -372,8 +372,16 @@
 #    define PTHREAD_GETSPECIFIC(key) pthread_getspecific(key)
 #endif
 
-#if defined(PERL_THREAD_LOCAL) && !defined(PERL_GET_CONTEXT) && !defined(PERL_SET_CONTEXT)
-/* Use C11 thread-local storage, where possible: */
+#if defined(PERL_THREAD_LOCAL) && !defined(PERL_GET_CONTEXT) && !defined(PERL_SET_CONTEXT) && !defined(__cplusplus)
+/* Use C11 thread-local storage, where possible.
+ * Frustratingly we can't (reliably) use it for C++ extensions, as some
+ * compilers on some platforms use one token for it for C, and a different token
+ * for it for C++, meaning that the working token that Configure probed for C
+ * turns out to be a compiler error on C++. Great.
+ * As Configure doesn't have a way to probe for C++ dialects, we just take the
+ * safe option and do the same as 5.34.0 and earlier - use pthreads on C++.
+ * Of course, if C++ XS extensions really want to avoid *all* this overhead,
+ * they should #define PERL_NO_GET_CONTEXT and pass aTHX/aTHX_ explicitly) */
 #  define PERL_USE_THREAD_LOCAL
 extern PERL_THREAD_LOCAL void *PL_current_context;