define NDEBUG in CMake non-debug builds

Summary:
This brings CMake builds further in line with builds that go through
the normal Makefile.
Closes https://github.com/facebook/rocksdb/pull/2300

Differential Revision: D5064631

Pulled By: yiwu-arbug

fbshipit-source-id: 7b2b2d5299f575f87badcf590cc95e040f14d52d

CMakeLists.txt

@@ -174,6 +174,7 @@ else()
   endif()
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
   if(NOT CMAKE_BUILD_TYPE STREQUAL "Debug")
+    add_definitions(-DNDEBUG)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -O2 -fno-omit-frame-pointer")
     include(CheckCXXCompilerFlag)
     CHECK_CXX_COMPILER_FLAG("-momit-leaf-frame-pointer" HAVE_OMIT_LEAF_FRAME_POINTER)