CMake: add support for SSE4.2

Summary: Closes https://github.com/facebook/rocksdb/pull/2159

Differential Revision: D4894483

Pulled By: yiwu-arbug

fbshipit-source-id: 607e17e8ef5d30dce02c27be9d1de7d9f823b4ae

CMakeLists.txt

@@ -115,6 +115,11 @@ if(WIN32)
       set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -mavx2")
     endif()
   endif()
+else()
+  option(WITH_SSE42 "build with SSE4.2" ON)
+  if(WITH_SSE42)
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -msse4.2")
+  endif()
 endif()
 
 set(BUILD_VERSION_CC ${CMAKE_BINARY_DIR}/build_version.cc)