Decouple C++ support in Meson's PGXS from LLVM enablement

Decouple C++ support in Meson's PGXS from LLVM enablement

This is important for Postgres extensions that are written in C++,
such as pg_duckdb, which uses PGXS as the build system currently.  In
the autotools build, C++ is not coupled to LLVM.  If the autotools
build is configured without --with-llvm, the C++ compiler and the
various flags get persisted into the Makefile.global.

Author: Tristan Partin <tristan@partin.io>
Author: Jelte Fennema-Nio <postgres@jeltef.nl>
Discussion: https://www.postgresql.org/message-id/flat/D98JHQF7H2A8.VSE3I4CJBTAB%40partin.io