simplify setting C++ standard

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5442bf7..f884fae 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -128,10 +128,6 @@
   add_required_cxx_compiler_flag(-m32)
 endif()
 
-set(CMAKE_CXX_STANDARD 11)
-set(CMAKE_CXX_STANDARD_REQUIRED ON)
-set(CMAKE_CXX_EXTENSIONS OFF)
-
 if (MSVC)
   set(BENCHMARK_CXX_STANDARD 14)
 else()