pgsql: 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(at)jeltef(dot)nl>
Co-authored-by: Peter Eisentraut <peter(at)eisentraut(dot)org>
Discussion: https://www.postgresql.org/message-id/flat/E1viDt1-001d7E-2I%40gemulon.postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ae4fe737aea36eb3a20e567c75cd70a82d1af230

Modified Files
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configure | 87 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++--
configure.ac | 41 +++++++++++++++++++++++++---
meson.build | 27 +++++++++++++++++++
3 files changed, 149 insertions(+), 6 deletions(-)