pythonPackages.{numpy,scipy,numexpr}: support MKL as BLAS

This adds support building with MKL.

pkgs/development/python-modules/numexpr/default.nix

@@ -3,6 +3,7 @@
 , fetchPypi
 , python
 , numpy
+, llvmPackages ? null
 }:
 
 buildPythonPackage rec {
@@ -14,6 +15,17 @@ buildPythonPackage rec {
     sha256 = "ee8bc7201aa2f1962c67d27c326a11eef9df887d7b87b1278a1d4e722bf44375";
   };
 
+  # Remove existing site.cfg, use the one we built for numpy.
+  # Somehow openmp needs to be added to LD_LIBRARY_PATH
+  # https://software.intel.com/en-us/forums/intel-system-studio/topic/611682
+  preBuild = ''
+    rm site.cfg
+    ln -s ${numpy.cfg} site.cfg
+    export LD_LIBRARY_PATH=${llvmPackages.openmp}/lib
+  '';
+
+  buildInputs = [] ++ lib.optional (numpy.blasImplementation == "mkl") llvmPackages.openmp;
+
   propagatedBuildInputs = [ numpy ];
 
   # Run the test suite.