Fix for C++ compatibility

Fix for C++ compatibility

After commit 476b35d4e31, some buildfarm members are complaining about
not recognizing _Noreturn when building the new C++ module
test_cplusplusext.  This is not a C++ feature, but it was gated like

    #if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L
    #define pg_noreturn _Noreturn

But apparently that was not sufficient.  Some platforms define
__STDC_VERSION__ even in C++ mode.  (In this particular case, it was
g++ on Solaris, but apparently this is also done by some other
platforms, and it is allowed by the C++ standard.)  To fix, add a

    ... && !defined(__cplusplus)

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/flat/CAGECzQR21OnnKiZO_1rLWO0-16kg1JBxnVq-wymYW0-_1cUNtg@mail.gmail.com