Simplify code related to compilation of SSL and OpenSSL

This commit makes more generic some comments and code related to the
compilation with OpenSSL and SSL in general to ease the addition of more
SSL implementations in the future.  In libpq, some OpenSSL-only code is
moved under USE_OPENSSL and not USE_SSL.

While on it, make a comment more consistent in libpq-fe.h.

Author: Daniel Gustafsson
Discussion: https://postgr.es/m/5382CB4A-9CF3-4145-BA46-C802615935E0@yesql.se

src/backend/libpq/hba.c

@@ -1041,7 +1041,7 @@ parse_hba_line(TokenizedLine *tok_line, int elevel)
 			ereport(elevel,
 					(errcode(ERRCODE_CONFIG_FILE_ERROR),
 					 errmsg("hostssl record cannot match because SSL is not supported by this build"),
-					 errhint("Compile with --with-ssl=openssl to use SSL connections."),
+					 errhint("Compile with --with-ssl to use SSL connections."),
 					 errcontext("line %d of configuration file \"%s\"",
 								line_num, HbaFileName)));
 			*err_msg = "hostssl record cannot match because SSL is not supported by this build";

src/include/libpq/libpq-be.h

@@ -194,7 +194,7 @@ typedef struct Port
 
 	/*
 	 * OpenSSL structures. (Keep these last so that the locations of other
-	 * fields are the same whether or not you build with OpenSSL.)
+	 * fields are the same whether or not you build with SSL enabled.)
 	 */
 #ifdef USE_OPENSSL
 	SSL		   *ssl;

src/include/pg_config_manual.h

@@ -176,8 +176,7 @@
 
 /*
  * USE_SSL code should be compiled only when compiling with an SSL
- * implementation.  (Currently, only OpenSSL is supported, but we might add
- * more implementations in the future.)
+ * implementation.
  */
 #ifdef USE_OPENSSL
 #define USE_SSL

src/interfaces/libpq/fe-secure.c

@@ -421,6 +421,13 @@ PQsslAttributeNames(PGconn *conn)
 
 	return result;
 }
+#endif							/* USE_SSL */
+
+/*
+ * Dummy versions of OpenSSL key password hook functions, when built without
+ * OpenSSL.
+ */
+#ifndef USE_OPENSSL
 
 PQsslKeyPassHook_OpenSSL_type
 PQgetSSLKeyPassHook_OpenSSL(void)
@@ -439,7 +446,7 @@ PQdefaultSSLKeyPassHook_OpenSSL(char *buf, int size, PGconn *conn)
 {
 	return 0;
 }
-#endif							/* USE_SSL */
+#endif							/* USE_OPENSSL */
 
 /* Dummy version of GSSAPI information functions, when built without GSS support */
 #ifndef ENABLE_GSS

src/interfaces/libpq/libpq-fe.h

@@ -619,7 +619,7 @@ extern int	pg_valid_server_encoding_id(int encoding);
 
 /* === in fe-secure-openssl.c === */
 
-/* Support for overriding sslpassword handling with a callback. */
+/* Support for overriding sslpassword handling with a callback */
 typedef int (*PQsslKeyPassHook_OpenSSL_type) (char *buf, int size, PGconn *conn);
 extern PQsslKeyPassHook_OpenSSL_type PQgetSSLKeyPassHook_OpenSSL(void);
 extern void PQsetSSLKeyPassHook_OpenSSL(PQsslKeyPassHook_OpenSSL_type hook);