Add maxscale/cdefs.h

The purpose of this file is to provide a header that *must* be included
first (that is, also before any system headers) by all other headers. On
the one hand this file provides a place where compilation environment
dependent things can be defined, and, on the other hand, a place where
things can be redefined globally, should that be necessary for whatever
reason. It further provides a place where constants applicable across
the line can be defined.

include/maxscale/cdefs.h

@@ -0,0 +1,38 @@
+#pragma once
+#ifndef _MAXSCALE_CDEFS_H
+#define _MAXSCALE_CDEFS_H
+/*
+ * 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/bsl.
+ *
+ * Change Date: 2019-07-01
+ *
+ * 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 cdefs.h
+ *
+ * This file has several purposes.
+ *
+ * - Its purpose is the same as that of x86_64-linux-gnu/sys/cdefs.h, that is,
+ *   it defines things that are dependent upon the compilation environment.
+ * - Since this *must* be included as the very first header by all other MaxScale
+ *   headers, it allows you to redfine things globally, should that be necessary,
+ *   for instance, when debugging something.
+ * - Global constants applicable across the line can be defined here.
+ */
+
+#ifdef	__cplusplus
+# define MXS_BEGIN_DECLS extern "C" {
+# define MXS_END_DECLS	 }
+#else
+# define MXS_BEGIN_DECLS
+# define MXS_END_DECLS
+#endif
+
+#endif