Adds more control for building MKL-DNN. (#322)

* Adds CMake variables `MKLDNN_BUILD_COMMAND_EXTRA_FLAGS`
  and `MKLDNN_CMAKE_EXTRA_FLAGS`.

cmake/external_mkldnn.cmake

@@ -17,6 +17,34 @@ include(ExternalProject)
 # Fetch and install MKL-DNN
 #----------------------------------------------------------------------------------------------------------
 
+set(MKLDNN_BUILD_COMMAND_EXTRA_FLAGS ""
+    CACHE STRING "Additional flags to supply to '${CMAKE_MAKE_PROGRAM}' when building MKLDNN."
+    )
+
+set(MKLDNN_CMAKE_EXTRA_FLAGS ""
+    CACHE STRING "Additional flags to supply to 'cmake' when building the MKLDNN build."
+    )
+
+# CMake is a terrible language when it comes to lists and strings.  Here's the behavior we *want*:
+#   1. CMake's user enters a space-separated list of additional command-line argments to be supplied
+#      to the 'make' invocation that builds MKL-DNN.
+#
+#   2. When that 'make' invocation occurs, each of thoise command-line arguments is a separate token
+#      on the command-line of the 'make' invocation.
+#
+# To avoid CMake grouping all of those command-line arguments together into a single, quote-
+# delimited string on the 'make' command-line, we need to temporarily convert the user-specified,
+# space-separated string into a CMake semicolon-separated list.
+separate_arguments(MKLDNN_BUILD_COMMAND_EXTRA_FLAGS_LIST UNIX_COMMAND
+    "${MKLDNN_BUILD_COMMAND_EXTRA_FLAGS}"
+    )
+
+separate_arguments(MKLDNN_CMAKE_EXTRA_FLAGS_LIST UNIX_COMMAND
+    "${MKLDNN_CMAKE_EXTRA_FLAGS}"
+    )
+
+#----------------------------------------------------------------------------------------------------------
+
 if(NOT ${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
 
     set(MKLDNN_GIT_REPO_URL https://github.com/01org/mkl-dnn)
@@ -32,7 +60,10 @@ if(NOT ${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
             UPDATE_COMMAND ""
             # Uncomment below with any in-flight MKL-DNN patches
             # PATCH_COMMAND patch -p1 < ${CMAKE_SOURCE_DIR}/third-party/patches/mkldnn-cmake-openmp.patch
-            CMAKE_ARGS -DCMAKE_INSTALL_PREFIX=${MKLDNN_INSTALL_DIR}
+            CMAKE_ARGS
+                -DCMAKE_INSTALL_PREFIX=${MKLDNN_INSTALL_DIR}
+                ${MKLDNN_CMAKE_EXTRA_FLAGS_LIST}
+            BUILD_COMMAND "${CMAKE_MAKE_PROGRAM}" ${MKLDNN_BUILD_COMMAND_EXTRA_FLAGS_LIST}
             )
     else()
         ExternalProject_Add(
@@ -42,8 +73,11 @@ if(NOT ${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
             UPDATE_COMMAND ""
             # Uncomment below with any in-flight MKL-DNN patches
             # PATCH_COMMAND patch -p1 < ${CMAKE_SOURCE_DIR}/third-party/patches/mkldnn-cmake-openmp.patch
-            CMAKE_ARGS -DCMAKE_INSTALL_PREFIX=${MKLDNN_INSTALL_DIR}
             BUILD_BYPRODUCTS "${MKLDNN_INSTALL_DIR}/include/mkldnn.hpp"
+            CMAKE_ARGS
+                -DCMAKE_INSTALL_PREFIX=${MKLDNN_INSTALL_DIR}
+                ${MKLDNN_CMAKE_EXTRA_FLAGS_LIST}
+            BUILD_COMMAND "${CMAKE_MAKE_PROGRAM}" ${MKLDNN_BUILD_COMMAND_EXTRA_FLAGS_LIST}
             )
     endif()