Document unusable PostgreSQL versions

An API break in PostgreSQL 10.4 and 9.6.9 makes it impossible
to use these versions: the "extract_actual_join_clauses" function
gained an additional parameter.

Document and add a safeguard to _PG_init.

Noticed by Francesco Pirotti in #272.

README.oracle_fdw

@@ -510,6 +510,9 @@ PostgreSQL 9.1 or better is required.
 Support for ANALYZE is available from PostgreSQL 9.2 on.  
 Support for INSERT, UPDATE and DELETE is available from PostgreSQL 9.3 on.
 
+Due to an API break in a PostgreSQL minor release, the following PostgreSQL
+versions cannot be used: 9.6.0 to 9.6.8 and 10.0 to 10.3
+
 oracle_fdw is written for standard open source PostgreSQL.
 Forks of PostgreSQL, such as "PostgresPro" and "Postgres-XL", are likely to
 be incompatible.

oracle_fdw.c

@@ -708,6 +708,20 @@ oracle_diag(PG_FUNCTION_ARGS)
 void
 _PG_init(void)
 {
+	/* check for incompatible server versions */
+	char *pgver_str = GetConfigOptionByName("server_version_num", NULL);
+	long pgver = strtol(pgver_str, NULL, 10);
+
+	pfree(pgver_str);
+
+	if ((pgver >= 90600 && pgver <= 90608)
+			|| (pgver >= 100000 && pgver <= 100003))
+		ereport(ERROR,
+				(errcode(ERRCODE_EXTERNAL_ROUTINE_INVOCATION_EXCEPTION),
+				errmsg("PostgreSQL version \"%s\" not supported by oracle_fdw",
+					   GetConfigOptionByName("server_version", NULL)),
+				errhint("You'll have to update PostgreSQL to a later minor release.")));
+
 	/* register an exit hook */
 	on_proc_exit(&exitHook, PointerGetDatum(NULL));
 }