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/statistics.h

@@ -0,0 +1,37 @@
+#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.
+ */
+
+/**
+ * @file
+ *
+ * Internal code for the statistics system.
+ */
+
+#include <maxscale/statistics.h>
+
+MXS_BEGIN_DECLS
+
+/**
+ * @brief Initialize statistics system
+ *
+ * This function should only be called once by the MaxScale core.
+ */
+void ts_stats_init();
+
+/**
+ * @brief Terminate statistics system
+ */
+void ts_stats_end();
+
+MXS_END_DECLS