Fix crash in Covariance if # threads > 1 requested without OpenMP.

- Previously if options.num_threads > 1 was given to Covariance compiled
  without OpenMP, a CHECK() would be triggered in program_evalutor.

Change-Id: Iaade4f5ed5326b0c59a7014c750c41ee026e1124

internal/ceres/covariance_impl.cc

diff --git a/internal/ceres/covariance_impl.cc b/internal/ceres/covariance_impl.cc
index 821be49..cfbfb44 100644
--- a/internal/ceres/covariance_impl.cc
+++ b/internal/ceres/covariance_impl.cc
@@ -79,8 +79,17 @@
     : options_(options),
       is_computed_(false),
       is_valid_(false) {
-  evaluate_options_.num_threads = options.num_threads;
-  evaluate_options_.apply_loss_function = options.apply_loss_function;
+#ifndef CERES_USE_OPENMP
+  if (options_.num_threads > 1) {
+    LOG(WARNING)
+        << "OpenMP support is not compiled into this binary; "
+        << "only options.num_threads = 1 is supported. Switching "
+        << "to single threaded mode.";
+    options_.num_threads = 1;
+  }
+#endif
+  evaluate_options_.num_threads = options_.num_threads;
+  evaluate_options_.apply_loss_function = options_.apply_loss_function;
 }
 
 CovarianceImpl::~CovarianceImpl() {