pgsql: docs: Document --with-selinux/-Dselinux options centrally

docs: Document --with-selinux/-Dselinux options centrally

Previously --with-selinux was documented only in the in the sepgsql
documentation and there was no corresponding documentation for meson. There
are further improvements that could be made, but this change seems worthwhile
even on its own.

Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Reported-by: Christoph Berg <myon@debian.org>
Discussion: https://postgr.es/m/20231103163848.26egkh5qdgw3vmil@awork3.anarazel.de

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/06c70849fb26ac431a722b1d10cffe1c65e728a4

Modified Files
--------------
doc/src/sgml/installation.sgml | 21 +++++++++++++++++++++
doc/src/sgml/sepgsql.sgml      | 11 ++++++++---
2 files changed, 29 insertions(+), 3 deletions(-)