separate benchmark target

CMakeLists.txt

@@ -18,8 +18,13 @@ if(WITH_UNIT_TESTS)
   enable_testing()
   include_directories(
       vendor/Catch/single_include
-      vendor/json.hpp/include
-      vendor/benchmark/include)
-  add_subdirectory(vendor/benchmark)
+      vendor/json.hpp/include)
   add_subdirectory(test)
 endif()
+
+if(WITH_BENCHMARK)
+  include_directories(
+      vendor/benchmark/include)
+  add_subdirectory(vendor/benchmark)
+  add_subdirectory(benchmark)
+endif()

benchmark/CMakeLists.txt

@@ -0,0 +1,2 @@
+add_executable(mstch_benchmark benchmark_main.cpp)
+target_link_libraries(mstch_benchmark mstch benchmark)