[PATCH] Fix Wundef warning for __STDC_VERSION__

[Siddhesh Poyarekar]

Hi,

c4c4124473c187b5c4642611390897666c3d3970 added the _Noreturn macro for
pre-C11 compilers, but it now throws a new Wundef warning during `make
check` for __STDC_VERSION__ which gcc does not define by default.  The
following patch fixes this in line with other uses of __STDC_VERSION__
in the file.

Siddhesh

	* misc/sys/cdefs.h: Check if __STDC_VERSION__ is defined
	before checking its value.

diff --git a/misc/sys/cdefs.h b/misc/sys/cdefs.h
index a5c3224..e6e4e0d 100644
--- a/misc/sys/cdefs.h
+++ b/misc/sys/cdefs.h
@@ -382,7 +382,8 @@
 # define __glibc_likely(cond)	(cond)
 #endif
 
-#if !defined _Noreturn && __STDC_VERSION__ < 201112 && !__GNUC_PREREQ (4,7)
+#if (!defined _Noreturn && defined __STDC_VERSION__ \
+     && __STDC_VERSION__ < 201112 && !__GNUC_PREREQ (4,7))
 # if __GNUC_PREREQ (2,8)
 #  define _Noreturn __attribute__ ((__noreturn__))
 # else
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