fftw_openmp

diff --git fftw/m4/ax_openmp.m4 fftw/m4/ax_openmp.m4
index 611ae0b..7981c0b 100644
--- fftw/m4/ax_openmp.m4
+++ fftw/m4/ax_openmp.m4
@@ -49,8 +49,18 @@ for ax_openmp_flag in $ax_openmp_flags; do
     none) []_AC_LANG_PREFIX[]FLAGS=$save[]_AC_LANG_PREFIX[] ;;
     *) []_AC_LANG_PREFIX[]FLAGS="$save[]_AC_LANG_PREFIX[]FLAGS $ax_openmp_flag" ;;
   esac
-  AC_TRY_LINK_FUNC(omp_set_num_threads,
-	[ax_cv_[]_AC_LANG_ABBREV[]_openmp=$ax_openmp_flag; break])
+  AC_TRY_LINK([#ifdef __cplusplus
+extern "C"
+#endif
+void omp_set_num_threads(int);], [const int N = 100000;
+  int i, arr[N];
+
+  omp_set_num_threads(2);
+
+  #pragma omp parallel for
+  for (i = 0; i < N; i++) {
+    arr[i] = i;
+  }], [ax_cv_[]_AC_LANG_ABBREV[]_openmp=$ax_openmp_flag; break])
 done
 []_AC_LANG_PREFIX[]FLAGS=$save[]_AC_LANG_PREFIX[]FLAGS
 ])