test-signalhandler.cpp: fixed -Wunused-macros warning on macOS

```
/Users/runner/work/cppcheck/cppcheck/test/signal/test-signalhandler.cpp:20:9: error: macro is not used [-Werror,-Wunused-macros]
   20 | #define _GNU_SOURCE // required to have feenableexcept()
      |         ^
```

Author: firewave

test/signal/test-signalhandler.cpp

@@ -16,9 +16,11 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+#if !defined(__APPLE__)
 #ifndef _GNU_SOURCE
 #define _GNU_SOURCE // required to have feenableexcept()
 #endif
+#endif
 
 #include "config.h"