Clang 11.0 编译的问题

1. src/bvar/scoped_timer.h
第50行
`void reset() { _start_time = butil::cpuwide_time_us(); }
`
错误，_start_time为const
`const int64_t _start_time;
`
2. brpc的CMakeLists.txt里，当cmake 版本大于3.1.3的时候（第125行），用了
```
set(CMAKE_CXX_STANDARD 11)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
```

这就造成我在cmake版本大于3.1.3的时候，无法修改生成的选项（例如自动生成-std=gnu++11，而我需要-std=c++11 -stdlib=libc++，选项不同在link其他库的时候，例如protobuf库如果是用-stdlib=libc++生成的话，brpc不加-stdlib=libc++会造成link error。

这里貌似没有什么必要区分cmake版本，

```
if(CMAKE_VERSION VERSION_LESS "3.1.3")
if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
endif()
if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
endif()
else()
set(CMAKE_CXX_STANDARD 11)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
endif()
```

直接修改成
`set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
`
应该没什么问题

Check src/bvar/scoped_timer.h around line 50 and brpc's CMakeLists.txt around line 125. Reproduce the Clang 11 build errors and inspect the existing CMake version handling. Done means the const-related compilation issue is resolved and users can select -std=c++11 and -stdlib=libc++ without incompatible generated flags.

Written by the indexing model from the issue text.