mv blas.h to src/misc such that it would be possible to use any blas libraries,

however, this requires some more works:
 - add const qualifiers in the declarations of blas.h
 - add the possibility to add a suffix to blas function names

Eigen/src/Core/util/MKL_support.h

@@ -53,9 +53,19 @@
 
 #include <mkl.h>
 #include <mkl_lapacke.h>
-
 #define EIGEN_MKL_VML_THRESHOLD 128
 
+#elif defined EIGEN_USE_BLAS
+
+#error Currently EIGEN_USE_BLAS requires Intel MKL. If you want to use MKL's BLAS and only it, then define EIGEN_USE_MKL too.
+
+#include "../../misc/blas.h"
+#define MKL_INT int
+
+#endif
+
+#if defined(EIGEN_USE_MKL) || defined(EIGEN_USE_BLAS)
+
 namespace Eigen {
 
 typedef std::complex<double> dcomplex;
@@ -68,6 +78,12 @@ static inline void assign_scalar_eig2mkl(MKLType& mklScalar, const EigenType& ei
   mklScalar=eigenScalar;
 }
 
+template<typename MKLType, typename EigenType>
+static inline void assign_conj_scalar_eig2mkl(MKLType& mklScalar, const EigenType& eigenScalar) {
+  mklScalar=eigenScalar;
+}
+
+#ifdef EIGEN_USE_MKL
 template <>
 inline void assign_scalar_eig2mkl<MKL_Complex16,dcomplex>(MKL_Complex16& mklScalar, const dcomplex& eigenScalar) {
   mklScalar.real=eigenScalar.real();
@@ -80,11 +96,6 @@ inline void assign_scalar_eig2mkl<MKL_Complex8,scomplex>(MKL_Complex8& mklScalar
   mklScalar.imag=eigenScalar.imag();
 }
 
-template<typename MKLType, typename EigenType>
-static inline void assign_conj_scalar_eig2mkl(MKLType& mklScalar, const EigenType& eigenScalar) {
-  mklScalar=eigenScalar;
-}
-
 template <>
 inline void assign_conj_scalar_eig2mkl<MKL_Complex16,dcomplex>(MKL_Complex16& mklScalar, const dcomplex& eigenScalar) {
   mklScalar.real=eigenScalar.real();
@@ -97,14 +108,12 @@ inline void assign_conj_scalar_eig2mkl<MKL_Complex8,scomplex>(MKL_Complex8& mklS
   mklScalar.imag=-eigenScalar.imag();
 }
 
+#endif
+
 } // end namespace internal
 
 } // end namespace Eigen
 
-#elif defined EIGEN_USE_BLAS
-
-#include "../../misc/blas.h"
-
 #endif
 
-#endif
+#endif // EIGEN_MKL_SUPPORT_H

Eigen/src/misc/blas.h

@@ -1,6 +1,11 @@
 #ifndef BLAS_H
 #define BLAS_H
 
+#ifdef __cplusplus
+extern "C"
+{
+#endif
+
 #define BLASFUNC(FUNC) FUNC##_
 
 #ifdef __WIN64__
@@ -646,4 +651,8 @@ int BLASFUNC(cpotri)(char *, int *, float  *, int *, int *);
 int BLASFUNC(zpotri)(char *, int *, double *, int *, int *);
 int BLASFUNC(xpotri)(char *, int *, double *, int *, int *);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif

blas/common.h

@@ -32,16 +32,8 @@
 #error the token SCALAR must be defined to compile this file
 #endif
 
-#ifdef __cplusplus
-extern "C"
-{
-#endif
+#include <Eigen/src/misc/blas.h>
 
-#include "blas.h"
-
-#ifdef __cplusplus
-}
-#endif
 
 #define NOTR    0
 #define TR      1