automatically add pthread option to autotune targets

this need cmake 3.1, if the cmake version should stay at 2.8 I could
find another way to link it

CMakeLists.txt

-cmake_minimum_required(VERSION 2.8.12)
+cmake_minimum_required(VERSION 3.1.3)
 project(dune-codegen CXX)
 
 if(NOT (dune-common_DIR OR dune-common_ROOT OR

python/CMakeLists.txt

@@ -24,6 +24,7 @@ add_subdirectory(test)
 add_executable(_autotune_target EXCLUDE_FROM_ALL _autotune.cc)
 target_compile_options(_autotune_target PUBLIC -fno-strict-aliasing)
 
-if(benchmark_FOUND)
-  target_link_libraries(_autotune_target benchmark)
+find_package(Threads)
+if(benchmark_FOUND AND Threads_FOUND)
+  target_link_libraries(_autotune_target benchmark Threads::Threads)
 endif()