Provide "eigen" defines to decide which instruction set is used

(sse3, ssse3 and sse4), independantly from the compiler.
Only those defines should be used in other places, and the user can
rely on those to know which sets are used.

Eigen/Core

@@ -61,20 +61,45 @@
 
 #ifndef EIGEN_DONT_VECTORIZE
   #if defined (EIGEN_SSE2_BUT_NOT_OLD_GCC) || defined(EIGEN_SSE2_ON_MSVC_2008_OR_LATER)
+
+    // Defines symbols for compile-time detection of which instructions are
+    // used.
+    // EIGEN_VECTORIZE_YY is defined if and only if the instruction set YY is used
     #define EIGEN_VECTORIZE
     #define EIGEN_VECTORIZE_SSE
-    #include <emmintrin.h>
-    #include <xmmintrin.h>
+    #define EIGEN_VECTORIZE_SSE2
+
+    // Detect sse3/ssse3/sse4:
+    // gcc and icc defines __SSE3__, ..,
+    // there is no way to know about this on msvc. You can define EIGEN_VECTORIZE_SSE* if you
+    // want to force the use of those instructions with msvc.
     #ifdef __SSE3__
-      #include <pmmintrin.h>
+      #define EIGEN_VECTORIZE_SSE3
     #endif
     #ifdef __SSSE3__
-      #include <tmmintrin.h>
+      #define EIGEN_VECTORIZE_SSSE3
     #endif
     #ifdef __SSE4_1__
-      #include <smmintrin.h>
+      #define EIGEN_VECTORIZE_SSE4_1
     #endif
     #ifdef __SSE4_2__
+      #define EIGEN_VECTORIZE_SSE4_2
+    #endif
+
+    // include files
+
+    #include <emmintrin.h>
+    #include <xmmintrin.h>
+    #ifdef  EIGEN_VECTORIZE_SSE3
+      #include <pmmintrin.h>
+    #endif
+    #ifdef EIGEN_VECTORIZE_SSSE3
+      #include <tmmintrin.h>
+    #endif
+    #ifdef EIGEN_VECTORIZE_SSE4_1
+      #include <smmintrin.h>
+    #endif
+    #ifdef EIGEN_VECTORIZE_SSE4_2
       #include <nmmintrin.h>
     #endif
   #elif defined __ALTIVEC__