pgsql: 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(at)partin(dot)io>
Author: Jelte Fennema-Nio <postgres(at)jeltef(dot)nl>
Discussion: https://www.postgresql.org/message-id/flat/D98JHQF7H2A8.VSE3I4CJBTAB%40partin.io

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/69d76fb2ab788d1715c82aece7b1dff10f483882

Modified Files
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meson.build | 29 ++++++++++++++++++-----------
src/include/meson.build | 8 +++++++-
src/makefiles/meson.build | 3 +--
3 files changed, 26 insertions(+), 14 deletions(-)