Update MKL instructions in the README.

Fix JuliaLang#5146

Make.inc

@@ -443,6 +443,9 @@ LIBMNAME = libimf
 endif
 
 ifeq ($(USE_MKL), 1)
+ifeq ($(USE_BLAS64), 1)
+export MKL_INTERFACE_LAYER := ILP64
+endif
 USE_SYSTEM_BLAS=1
 USE_SYSTEM_LAPACK=1
 LIBBLASNAME = libmkl_rt

README.md

@@ -225,12 +225,18 @@ SuiteSparse is a special case, since it is typically only installed as a static
 
 ### Intel Math Kernel Libraries
 
-To use the Intel [MKL] BLAS and LAPACK libraries, edit the following settings in `Make.inc`:
+To use the Intel [MKL] BLAS and LAPACK libraries, make sure that MKL version 10.3.6 or higher is installed, and the MKL environment is set up:
+
+ source /path/to/mkl/bin/mklvars.sh
+
+On a 64-bit computer, ensure that the ILP64 version of MKL is used:
+
+ export MKL_INTERFACE_LAYER=ILP64
+
+When building julia, pass the `USE_MKL=1` option to `make` or add the following line to `Make.user`.
 
  USE_MKL = 1
- MKLLIB = /path/to/mkl/lib/arch
 
-`MKLLIB` points to the directory containing `libmkl_rt.so`. MKL version 10.3.6 or greater is required.
 To rebuild a pre-built Julia source install with MKL support, delete the OpenBLAS, ARPACK, and SuiteSparse dependencies from `deps`, and run `make cleanall testall`.
 
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