Cygwin: fix CPU_SET macro visibility

The CPU_SET macros defined in Cygwin's include/sys/cpuset.h must not
be visible in an application's namespace unless _GNU_SOURCE has been
#defined.  Internally this means wrapping them in #if __GNU_VISIBLE.

winsup/cygwin/include/sys/cpuset.h

@@ -26,6 +26,7 @@ typedef struct
   __cpu_mask __bits[__CPU_GROUPMAX];
 } cpu_set_t;
 
+#if __GNU_VISIBLE
 int __sched_getaffinity_sys (pid_t, size_t, cpu_set_t *);
 
 /* These macros alloc or free dynamically-sized cpu sets of size 'num' cpus.
@@ -88,6 +89,8 @@ int __sched_getaffinity_sys (pid_t, size_t, cpu_set_t *);
 #define CPU_XOR(dst, src1, src2)  CPU_XOR_S(sizeof (cpu_set_t), dst, src1, src2)
 #define CPU_EQUAL(src1, src2)     CPU_EQUAL_S(sizeof (cpu_set_t), src1, src2)
 
+#endif /* __GNU_VISIBLE */
+
 #ifdef __cplusplus
 }
 #endif