Fix in-source builds

The internal header directory conflicted with in-source builds causing a
build failure. This is fixed by renaming the internal header directory to
something other than maxscale.

The renaming pointed out a few problems in a couple of source files that
appeared to include internal headers when the headers were in fact public
headers.

Fixed maxctrl in-source builds by making the copying of the sources
optional.

server/core/internal/query_classifier.h

@@ -0,0 +1,39 @@
+#pragma once
+/*
+ * Copyright (c) 2016 MariaDB Corporation Ab
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file and at www.mariadb.com/bsl11.
+ *
+ * Change Date: 2020-01-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2 or later of the General
+ * Public License.
+ */
+
+#include <maxscale/cdefs.h>
+#include <maxscale/query_classifier.h>
+
+MXS_BEGIN_DECLS
+
+typedef enum qc_trx_parse_using
+{
+    QC_TRX_PARSE_USING_QC,     /**< Use the query classifier. */
+    QC_TRX_PARSE_USING_PARSER, /**< Use custom parser. */
+} qc_trx_parse_using_t;
+
+/**
+ * Returns the type bitmask of transaction related statements.
+ *
+ * @param stmt  A COM_QUERY or COM_STMT_PREPARE packet.
+ * @param use   What method should be used.
+ *
+ * @return The relevant type bits if the statement is transaction
+ *         related, otherwise 0.
+ *
+ * @see qc_get_trx_type_mask
+ */
+uint32_t qc_get_trx_type_mask_using(GWBUF* stmt, qc_trx_parse_using_t use);
+
+MXS_END_DECLS