Fix mkldnn build bug in Ubuntu

cmake/external/mkldnn.cmake

@@ -57,6 +57,8 @@ ExternalProject_Add(
     GIT_TAG             "a29d8487a63afca3d5b8c5bbdbb473cf8ccc6e51"
     PREFIX              ${MKLDNN_SOURCES_DIR}
     UPDATE_COMMAND      ""
+    CMAKE_ARGS          -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}
+    CMAKE_ARGS          -DCMAKE_C_COMPILER=${CMAKE_C_COMPILER}
     CMAKE_ARGS          -DCMAKE_INSTALL_PREFIX=${MKLDNN_INSTALL_DIR}
     CMAKE_ARGS          -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}
     CMAKE_ARGS          -DMKLROOT=${MKLML_ROOT}