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

This adds support building with MKL.

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

@@ -1,6 +1,20 @@
-{ stdenv, lib, fetchPypi, python, buildPythonPackage, isPyPy, gfortran, pytest, blas }:
+{ stdenv, lib, fetchPypi, python, buildPythonPackage, isPyPy, gfortran, pytest, blas, writeTextFile }:
 
-buildPythonPackage rec {
+let
+  blasImplementation = lib.nameFromURL blas.name "-";
+  cfg = writeTextFile {
+    name = "site.cfg";
+    text = (lib.generators.toINI {} {
+      "${blasImplementation}" = {
+        include_dirs = "${blas}/include";
+        library_dirs = "${blas}/lib";
+      } // lib.optionalAttrs (blasImplementation == "mkl") {
+        mkl_libs = "mkl_rt";
+        lapack_libs = "";
+      };
+    });
+  };
+in buildPythonPackage rec {
   pname = "numpy";
   version = "1.15.2";
 
@@ -39,12 +53,7 @@ buildPythonPackage rec {
   '';
 
   preBuild = ''
-    echo "Creating site.cfg file..."
-    cat << EOF > site.cfg
-    [openblas]
-    include_dirs = ${blas}/include
-    library_dirs = ${blas}/lib
-    EOF
+    ln -s ${cfg} site.cfg
   '';
 
   enableParallelBuilding = true;
@@ -59,8 +68,11 @@ buildPythonPackage rec {
 
   passthru = {
     blas = blas;
+    inherit blasImplementation cfg;
   };
 
+  doCheck = blasImplementation != "mkl";
+
   # Disable two tests
   # - test_f2py: f2py isn't yet on path.
   # - test_large_file_support: takes a long time and can cause the machine to run out of disk space