configure: Apply -Werror=vla to C++ as well as C

configure: Apply -Werror=vla to C++ as well as C

The comment part of d9dd406fe281 mentioned that -Wvla is not applicable
for C++. That is not fully correct: it is true that VLAs are not part of the
C++ standard, and g++ with -pedantic will also warn about them as a non-standard
extension.  However, -Wvla itself works fine in C++ and will catch VLA
usage just as in C.

Fix configure.ac to apply -Werror=vla to C++ as well. There is no need to
fix meson.build as it already includes it in common_warning_flags.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Suggested-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/7bf60ab1-2b5d-4a77-93ce-815072a0a014%40eisentraut.org