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

doc/src/sgml/installation.sgml

@@ -1198,6 +1198,16 @@ build-postgresql:
        </listitem>
       </varlistentry>
 
+      <varlistentry id="configure-option-with-sepgsql">
+       <term><option>--with-selinux</option></term>
+       <listitem>
+        <para>
+         Build with SElinux support, enabling the <xref linkend="sepgsql"/>
+         extension.
+        </para>
+       </listitem>
+      </varlistentry>
+
      </variablelist>
 
    </sect3>
@@ -2629,6 +2639,17 @@ ninja install
        </para>
       </listitem>
      </varlistentry>
+
+     <varlistentry id="configure-with-sepgsql-meson">
+      <term><option>-Dselinux={ auto | enabled | disabled }</option></term>
+      <listitem>
+       <para>
+        Build with SElinux support, enabling the <xref linkend="sepgsql"/>
+        extension.  Defaults to auto.
+       </para>
+      </listitem>
+     </varlistentry>
+
     </variablelist>
    </sect3>
 

doc/src/sgml/sepgsql.sgml

@@ -87,9 +87,14 @@ Policy from config file:        targeted
   </para>
 
   <para>
-   To build this module, include the option <literal>--with-selinux</literal> in
-   your PostgreSQL <literal>configure</literal> command.  Be sure that the
-   <filename>libselinux-devel</filename> RPM is installed at build time.
+   To build this module specify <xref
+   linkend="configure-option-with-sepgsql"/> (when using <link
+   linkend="install-make">make and autoconf</link> ) or <xref
+   linkend="configure-with-sepgsql-meson"/> (when using <link
+   linkend="install-meson">meson</link>).
+
+   Be sure that the <filename>libselinux-devel</filename> RPM is installed at
+   build time.
   </para>
 
   <para>