Don't require OpenMP in sort creatures 2d example

examples/soft_creatures_2d.cpp

@@ -751,7 +751,9 @@ private:
 
 			std::vector<creature> evaluated_creatures(population_.size());
 
+#ifdef _OPENMP
 			#pragma omp parallel for
+#endif
 			for (int i = 0; i < population_.size(); ++i)
 			{
 				auto & creature = population_[i];