Support system Google Benchmark

apps/benchmarks/CMakeLists.txt

@@ -1,3 +1,8 @@
+option(OPENMW_USE_SYSTEM_BENCHMARK "Use system Google Benchmark library." OFF)
+
+if(OPENMW_USE_SYSTEM_BENCHMARK)
+    find_package(benchmark REQUIRED)
+else()
     cmake_minimum_required(VERSION 3.11)
 
     set(BENCHMARK_ENABLE_TESTING OFF)
@@ -17,6 +22,7 @@ FetchContent_Declare(benchmark
     FetchContent_MakeAvailableExcludeFromAll(benchmark)
 
     set(CMAKE_CXX_FLAGS "${SAVED_CMAKE_CXX_FLAGS}")
+endif()
 
 openmw_add_executable(openmw_detournavigator_navmeshtilescache_benchmark detournavigator/navmeshtilescache.cpp)
 target_compile_features(openmw_detournavigator_navmeshtilescache_benchmark PRIVATE cxx_std_17)