pgsql: Support using copyObject in standard C++

Support using copyObject in standard C++

Calling copyObject in C++ without GNU extensions (e.g. when using
-std=c++11 instead of -std=gnu++11) fails with an error like this:

error: use of undeclared identifier 'typeof'; did you mean 'typeid'

This is due to the C compiler used to compile PostgreSQL supporting
typeof, but that function actually not being present in the C++
compiler. This fixes that by explicitely checking for typeof support
in C++, and then either use that or define typeof ourselves as:

std::remove_reference_t<decltype(x)>

According to the paper that led to adding typeof to the C standard,
that's the C++ equivalent of the C typeof:
https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2927.htm#existing-decltype

Author: Author: Jelte Fennema-Nio <postgres(at)jeltef(dot)nl>
Discussion: https://www.postgresql.org/message-id/flat/DGPW5WCFY7WY.1IHCDNIVVT300%2540jeltef.nl

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/1887d822f1428637c935b9f5e8be4a2a56fb77c7

Modified Files
--------------
config/c-compiler.m4 | 29 ++++++++++++
configure | 54 ++++++++++++++++++++++
configure.ac | 1 +
meson.build | 25 ++++++++++
src/include/c.h | 21 +++++++++
src/include/pg_config.h.in | 7 +++
.../test_cplusplusext/test_cplusplusext.cpp | 2 +
7 files changed, 139 insertions(+)