Fix USE_MKLDNN check in Makefile (apache#13775)

* fix makefile

* change make/config.mk

* add comments

* retrigger ci

Author: TaoLv

Makefile

@@ -61,7 +61,7 @@ endif
 # use customized config file
 include $(config)
 
-ifndef $(USE_MKLDNN)
+ifndef USE_MKLDNN
 ifneq ($(UNAME_S), Darwin)
 ifneq ($(UNAME_S), Windows)
 ifeq ($(UNAME_P), x86_64)

make/config.mk

@@ -98,8 +98,10 @@ USE_LIBJPEG_TURBO_PATH = NONE
 # use openmp for parallelization
 USE_OPENMP = 1
 
-# whether use MKL-DNN library
-USE_MKLDNN = 0
+# whether use MKL-DNN library: 0 = disabled, 1 = enabled
+# if USE_MKLDNN is not defined, MKL-DNN will be enabled by default on x86 Linux.
+# you can disable it explicity with USE_MKLDNN = 0
+USE_MKLDNN =
 
 # whether use NNPACK library
 USE_NNPACK = 0