compiler: Use C11 build assertions with new enough GCC or Clang.

Until now, the BUILD_ASSERT and BUILD_ASSERT_DECL macros have used OVS's
home-grown build assertion strategy.  This commit switches them to using
C11 build assertions with compilers that support them.  The semantics are
the same, but C11 build assertions yield clearer error messages when they
fail.

This commit also reorders the definitions a bit to make it easier to
follow.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Jarno Rajahalme <jarno@ovn.org>

include/openvswitch/compiler.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2016 Nicira, Inc.
+ * Copyright (c) 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2016, 2017 Nicira, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -236,26 +236,28 @@
 #define OVS_PREFETCH_WRITE(addr)
 #endif
 
-/* Build assertions. */
+/* Build assertions.
+ *
+ * Use BUILD_ASSERT_DECL as a declaration or a statement, or BUILD_ASSERT as
+ * part of an expression. */
 #ifdef __CHECKER__
 #define BUILD_ASSERT(EXPR) ((void) 0)
 #define BUILD_ASSERT_DECL(EXPR) extern int (*build_assert(void))[1]
-#elif !defined(__cplusplus)
-/* Build-time assertion building block. */
+#elif defined(__cplusplus)
+#include <boost/static_assert.hpp>
+#define BUILD_ASSERT BOOST_STATIC_ASSERT
+#define BUILD_ASSERT_DECL BOOST_STATIC_ASSERT
+#elif (__GNUC__ * 256 + __GNUC_MINOR__ >= 0x403 \
+       || __has_extension(c_static_assert))
+#define BUILD_ASSERT_DECL(EXPR) _Static_assert(EXPR, #EXPR)
+#define BUILD_ASSERT(EXPR) (void) ({ _Static_assert(EXPR, #EXPR); })
+#else
 #define BUILD_ASSERT__(EXPR) \
         sizeof(struct { unsigned int build_assert_failed : (EXPR) ? 1 : -1; })
-
-/* Build-time assertion for use in a statement context. */
 #define BUILD_ASSERT(EXPR) (void) BUILD_ASSERT__(EXPR)
-
-/* Build-time assertion for use in a declaration context. */
 #define BUILD_ASSERT_DECL(EXPR) \
         extern int (*build_assert(void))[BUILD_ASSERT__(EXPR)]
-#else /* __cplusplus */
-#include <boost/static_assert.hpp>
-#define BUILD_ASSERT BOOST_STATIC_ASSERT
-#define BUILD_ASSERT_DECL BOOST_STATIC_ASSERT
-#endif /* __cplusplus */
+#endif
 
 #ifdef __GNUC__
 #define BUILD_ASSERT_GCCONLY(EXPR) BUILD_ASSERT(EXPR)