Detect if flags are needed for C++11 support

Detect if flags are needed for C++11 support

Just like we only support compiling with C11, we only support
compiling extensions with C++11 and up.  Some compilers support C++11
but don't enable it by default.  This detects if flags are needed to
enable C++11 support, in a similar way to how we check the same for
C11 support.

The C++ test extension module added by commit 476b35d4e31 confirmed
that C++11 is effectively required.  (This was understood in mailing
list discussions but not recorded anywhere in the source code.)

Author: Jelte Fennema-Nio <postgres@jeltef.nl>
Co-authored-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://www.postgresql.org/message-id/flat/E1viDt1-001d7E-2I%40gemulon.postgresql.org