Guard #define __STDC_NO_ATOMICS__ by #ifndef

[Bodigrim]

The following error was reported at https://www.reddit.com/r/haskell/comments/1nrt23b/comment/ngka3xp/:

error: <command-line>: warning: "__STDC_NO_ATOMICS__" redefined 

I'm not really sure under which circumstances it could happen (the Reddit thread is about using GHC on a Solaris-family OS), but we can certainly guard #define __STDC_NO_ATOMICS__ by #ifndef __STDC_NO_ATOMICS__ in

text/cbits/measure_off.c

Lines 17 to 23 in 86b5590

	#if !(__GNUC__ >= 5 || __GNUC__ == 4 && __GNUC_MINOR__ >= 9 || defined(__clang_major__))
	#define __STDC_NO_ATOMICS__
	#endif
	#ifndef __STDC_NO_ATOMICS__
	#include <stdatomic.h>
	#endif