sys/cdefs.h: move __BEGIN_DECLS/__END_DECLS into a helper sys/_decls.h

There are situations where nothing from sys/cdefs.h is needed except for
the declaration braces.  More, the other facilities from sys/cdefs.h
might unnecessarly pollute the namespace.

Reviewed by:	markj, imp
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D56889

sys/sys/_decls.h

@@ -0,0 +1,46 @@
+/*-
+ * SPDX-License-Identifier: BSD-3-Clause
+ *
+ * Copyright (c) 1991, 1993
+ *	The Regents of the University of California.  All rights reserved.
+ *
+ * This code is derived from software contributed to Berkeley by
+ * Berkeley Software Design, Inc.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. Neither the name of the University nor the names of its contributors
+ *    may be used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#ifndef _SYS__DECLS_H_
+#define _SYS__DECLS_H_
+
+#if defined(__cplusplus)
+#define	__BEGIN_DECLS	extern "C" {
+#define	__END_DECLS	}
+#else
+#define	__BEGIN_DECLS
+#define	__END_DECLS
+#endif
+
+#endif

sys/sys/cdefs.h

@@ -61,13 +61,7 @@
 #define	__has_builtin(x)	0
 #endif
 
-#if defined(__cplusplus)
-#define	__BEGIN_DECLS	extern "C" {
-#define	__END_DECLS	}
-#else
-#define	__BEGIN_DECLS
-#define	__END_DECLS
-#endif
+#include <sys/_decls.h>
 
 /*
  * This code has been put in place to help reduce the addition of

tools/build/Makefile

@@ -360,6 +360,7 @@ DISKINCS+=	${SRCTOP}/sys/sys/disk/bsd.h
 .if ${.MAKE.OS} == "FreeBSD"
 # Needed to build most of the things below, which include sys/cdefs.h either
 # directly or transitively
+SYSINCS+=	${SRCTOP}/sys/sys/_decls.h
 SYSINCS+=	${SRCTOP}/sys/sys/_visible.h
 SYSINCS+=	${SRCTOP}/sys/sys/cdefs.h
 .endif