mxnet: fix library dependencies.

- mxnet now builds the open source MKL DNN library. However, this
  was not included with the build.
- mxnet builds its own OpenMP library, even when OpenMP is provided
  by the system. This leads to problems, so remove the included
  OpenMP implementation:

  https://github.com/apache/incubator-mxnet/pull/12160

pkgs/applications/science/math/mxnet/default.nix

@@ -34,11 +34,14 @@ stdenv.mkDerivation rec {
   postPatch = ''
     substituteInPlace 3rdparty/mkldnn/tests/CMakeLists.txt \
       --replace "/bin/bash" "${bash}/bin/bash"
+
+    # Build against the system version of OpenMP. 
+    # https://github.com/apache/incubator-mxnet/pull/12160
+    rm -rf 3rdparty/openmp
   '';
 
-  installPhase = ''
-    install -Dm755 libmxnet.so $out/lib/libmxnet.so
-    cp -r ../include $out
+  postInstall = ''
+    rm "$out"/lib/*.a
   '';
 
   enableParallelBuilding = true;