include/starpu_util.h: fix code for cases when _GNU_SOURCE is defined without a value

include/starpu_util.h

@@ -159,7 +159,7 @@ extern "C"
 } while(0)
 
 #if defined(STARPU_HAVE_STRERROR_R)
-#if !__GLIBC__ || ((_POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600) && ! _GNU_SOURCE)
+#if !__GLIBC__ || ((_POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600) && ! defined(_GNU_SOURCE))
 /* XSI-compliant version of strerror_r returns an int */
 #define starpu_strerror_r(errnum, buf, buflen) \
 	do \