Introduce --with-ssl={openssl} as a configure option

This is a replacement for the existing --with-openssl, extending the
logic to make easier the addition of new SSL libraries.  The grammar is
chosen to be similar to --with-uuid, where multiple values can be
chosen, with "openssl" as the only supported value for now.

The original switch, --with-openssl, is kept for compatibility.

Author: Daniel Gustafsson, Michael Paquier
Reviewed-by: Jacob Champion
Discussion: https://postgr.es/m/FAB21FC8-0F62-434F-AA78-6BD9336D630A@yesql.se

doc/src/sgml/installation.sgml

@@ -967,7 +967,7 @@ build-postgresql:
       </varlistentry>
 
       <varlistentry>
-       <term><option>--with-openssl</option>
+       <term><option>--with-ssl=<replaceable>LIBRARY</replaceable></option>
        <indexterm>
         <primary>OpenSSL</primary>
         <seealso>SSL</seealso>
@@ -976,11 +976,22 @@ build-postgresql:
        <listitem>
         <para>
          Build with support for <acronym>SSL</acronym> (encrypted)
-         connections. This requires the <productname>OpenSSL</productname>
-         package to be installed.  <filename>configure</filename> will check
-         for the required header files and libraries to make sure that
-         your <productname>OpenSSL</productname> installation is sufficient
-         before proceeding.
+         connections. The only <replaceable>LIBRARY</replaceable>
+         supported is <option>openssl</option>. This requires the
+         <productname>OpenSSL</productname> package to be installed.
+         <filename>configure</filename> will check for the required
+         header files and libraries to make sure that your
+         <productname>OpenSSL</productname> installation is sufficient
+         before proceeding. 
+        </para>
+       </listitem>
+      </varlistentry>
+
+      <varlistentry>
+       <term><option>--with-openssl</option></term>
+       <listitem>
+        <para>
+         Obsolete equivalent of <literal>--with-ssl=openssl</literal>.
         </para>
        </listitem>
       </varlistentry>

doc/src/sgml/pgcrypto.sgml

@@ -1154,7 +1154,7 @@ gen_random_uuid() returns uuid
     <filename>pgcrypto</filename> configures itself according to the findings of the
     main PostgreSQL <literal>configure</literal> script.  The options that
     affect it are <literal>--with-zlib</literal> and
-    <literal>--with-openssl</literal>.
+    <literal>--with-ssl=openssl</literal>.
    </para>
 
    <para>

doc/src/sgml/sslinfo.sgml

@@ -22,7 +22,7 @@
 
  <para>
   This extension won't build at all unless the installation was
-  configured with <literal>--with-openssl</literal>.
+  configured with <literal>--with-ssl=openssl</literal>.
  </para>
 
  <sect2>