Makefile plugins target needs mkldnn dependency

[samskalicky]

Description

Building MXNet from source and enabling the WarpCTC plugin following these instructions:
https://github.com/apache/incubator-mxnet/tree/master/example/ctc#ctc-loss-in-mxnet
fails to build on Ubuntu with make -j$nproc because it depends on mkldnn which wasnt compiled yet. This is due to a missing makefile dependency on the plugin build target.

Heres the error message:

In file included from include/mxnet/./op_attr_types.h:36:0,
                 from include/mxnet/operator.h:40,
                 from plugin/warpctc/./warpctc-inl.h:31,
                 from plugin/warpctc/warpctc.cc:27:
include/mxnet/././ndarray.h:41:22: fatal error: mkldnn.hpp: No such file or directory
 #include <mkldnn.hpp>
                      ^
compilation terminated.
make: *** [build/plugin/warpctc/warpctc.o] Error 1

[mxnet-label-bot]

Hey, this is the MXNet Label Bot.
Thank you for submitting the issue! I will try and suggest some labels so that the appropriate MXNet community members can help resolve it.
Here are my recommended labels: Build

[frankfliu]

@mxnet-label-bot add [Build, Bug]

[pengzhao-intel]

Thanks to raising the issue. @xinyu-intel could you help take a look?

[pengzhao-intel]

@mxnet-label-bot add [MKLDNN]

[rongzha1]

@samskalicky you can workaround by using: make -j1

[samskalicky]

Thanks guys!

Ive tried adding " | mkldnn " to the Makefile here:

https://github.com/apache/incubator-mxnet/blob/master/Makefile#L492

that seems to be working. This is similar to another target above:

https://github.com/apache/incubator-mxnet/blob/master/Makefile#L476

Any thoughts about committing this change into the makefile?

[apeforest]

Why do we need warpctc if mxnet already has native implementation?