fixed linker error caused when not using --with-darshan flag

configure.ac

@@ -97,7 +97,7 @@ if test "x${with_darshan}" != "x" ; then
     AC_CHECK_FILE([$darshan_headers/darshan-io-events.h],
                   AM_CONDITIONAL(USE_DARSHAN, true),
                   AC_MSG_ERROR(Could not find darshan-io-events.h at: $darshan_headers))
-    DARSHAN_CPPFLAGS="-I${with_darshan}/include"
+    DARSHAN_CPPFLAGS="-DUSE_DARSHAN=1 -I${with_darshan}/include"
     AC_SUBST(DARSHAN_CPPFLAGS)
 else
     AM_CONDITIONAL(USE_DARSHAN, false)

src/workload/codes-workload.c

@@ -15,10 +15,17 @@
  */ 
 extern struct codes_workload_method test_workload_method;
 extern struct codes_workload_method bgp_io_workload_method;
+#ifdef USE_DARSHAN
 extern struct codes_workload_method darshan_io_workload_method;
-
+#endif
 static struct codes_workload_method *method_array[] = 
-    {&test_workload_method, &bgp_io_workload_method, &darshan_io_workload_method, NULL};
+{
+    &test_workload_method,
+    &bgp_io_workload_method,
+#ifdef USE_DARSHAN
+    &darshan_io_workload_method,
+#endif
+    NULL};
 
 /* This shim layer is responsible for queueing up reversed operations and
  * re-issuing them so that the underlying workload generator method doesn't