[newlib-cygwin] Make _Static_assert() work with GCC in older C++ standards.

[newlib-cygwin] Make _Static_assert() work with GCC in older C++ standards.

[Sebastian Huber]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=84e294dc694257d8f3e9b00a4a0651afbf76132a

commit 84e294dc694257d8f3e9b00a4a0651afbf76132a
Author: ed@FreeBSD.org <ed@FreeBSD.org>
Date:   Mon Aug 28 09:35:17 2017 +0000

    Make _Static_assert() work with GCC in older C++ standards.
    
    GCC only activates C11 keywords in C mode, not C++ mode. This means
    that when targeting an older C++ standard, we cannot fall back to using
    _Static_assert(). In this case, do define _Static_assert() as a macro
    that uses a typedef'ed array.
    
    Discussed in:	r322875 commit thread
    Reported by:	Mark MIllard
    MFC after:	1 month

Diff:
---
 newlib/libc/include/sys/cdefs.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/newlib/libc/include/sys/cdefs.h b/newlib/libc/include/sys/cdefs.h
index b3f8d19..49be9e0 100644
--- a/newlib/libc/include/sys/cdefs.h
+++ b/newlib/libc/include/sys/cdefs.h
@@ -314,7 +314,7 @@
 #if (defined(__cplusplus) && __cplusplus >= 201103L) || \
     __has_extension(cxx_static_assert)
 #define	_Static_assert(x, y)	static_assert(x, y)
-#elif __GNUC_PREREQ__(4,6)
+#elif __GNUC_PREREQ__(4,6) && !defined(__cplusplus)
 /* Nothing, gcc 4.6 and higher has _Static_assert built-in */
 #elif defined(__COUNTER__)
 #define	_Static_assert(x, y)	__Static_assert(x, __COUNTER__)