Guard parallel_for_nothreads with CERES_NO_THREADS.

- Raised as issue #384.

Change-Id: I72954d93ce3f27b58b719b28b766c7c0e7a6faf6

internal/ceres/parallel_for_nothreads.cc

diff --git a/internal/ceres/parallel_for_nothreads.cc b/internal/ceres/parallel_for_nothreads.cc
index 63b8f2a..73f6d30 100644
--- a/internal/ceres/parallel_for_nothreads.cc
+++ b/internal/ceres/parallel_for_nothreads.cc
@@ -31,6 +31,8 @@
 // This include must come before any #ifndef check on Ceres compile options.
 #include "ceres/internal/port.h"
 
+#ifdef CERES_NO_THREADS
+
 #include "ceres/parallel_for.h"
 #include "glog/logging.h"
 
@@ -70,3 +72,5 @@
 
 }
 }
+
+#endif  // CERES_NO_THREADS