CMake ignores USE_MKLDNN flag for 1.2.0.RC[0-2] releases

[anirudh2290]

CMake ignores the USE_MKLDNN flag for 1.2.RC2 release. This has been fixed in the master as part of #10731. To workaround the issue on 1.2.RC2 release, please apply the following changes in CMakeLists.txt:

 if(USE_MKL_IF_AVAILABLE)
   if(USE_MKLDNN)
+    set(ARCH_OPT_FLAGS "-mtune=generic")
     add_subdirectory(3rdparty/mkldnn)
     include_directories(3rdparty/mkldnn/include)
+    add_definitions(-DMXNET_USE_MKLDNN=1)
     list(APPEND mxnet_LINKER_LIBS mkldnn)
   endif()
   find_package(MKL)

[roywei]

@anirudh2290 could you add relevant labels (installation/build/MKL) to keep track? Thanks