Do not check MaxNumThreadsAvailable if the thread number is set to 1.

- this saves the sysfs cost in MaxNumThreadsAvailable.

Change-Id: I6b33ceae363da19346341be95abbd89d7b7b0f2f

internal/ceres/preprocessor.cc

diff --git a/internal/ceres/preprocessor.cc b/internal/ceres/preprocessor.cc
index 6a67d38..cd8c48c 100644
--- a/internal/ceres/preprocessor.cc
+++ b/internal/ceres/preprocessor.cc
@@ -57,6 +57,9 @@
 Preprocessor::~Preprocessor() {}
 
 void ChangeNumThreadsIfNeeded(Solver::Options* options) {
+  if (options->num_threads == 1) {
+    return;
+  }
   const int num_threads_available = MaxNumThreadsAvailable();
   if (options->num_threads > num_threads_available) {
     LOG(WARNING) << "Specified options.num_threads: " << options->num_threads