CMake: compile with -O2

Summary:
Seems pretty important.
Closes https://github.com/facebook/rocksdb/pull/2161

Differential Revision: D4891097

Pulled By: yiwu-arbug

fbshipit-source-id: 16dc0149dc89d426407fabab4a5ba1d6d1562269

CMakeLists.txt

@@ -138,7 +138,7 @@ else()
   endif()
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
   if(NOT CMAKE_BUILD_TYPE STREQUAL "Debug")
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-omit-frame-pointer")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -O2 -fno-omit-frame-pointer")
     include(CheckCXXCompilerFlag)
     CHECK_CXX_COMPILER_FLAG("-momit-leaf-frame-pointer" HAVE_OMIT_LEAF_FRAME_POINTER)
     if(HAVE_OMIT_LEAF_FRAME_POINTER)