let the user disable alignment altogether by #defining EIGEN_DONT_ALIGN.

Until now, the user had to edit the source code to do that.
Internally, add EIGEN_ALIGN that takes into account both EIGEN_DONT_ALIGN.and
EIGEN_ARCH_WANTS_ALIGNMENT. From now on, only EIGEN_ALIGN should be used to
test whether we want to align.

Eigen/Core

@@ -27,6 +27,10 @@
   #define EIGEN_SSE2_BUT_NOT_OLD_GCC
 #endif
 
+#ifdef EIGEN_DONT_ALIGN
+  #define EIGEN_DONT_VECTORIZE
+#endif
+
 #ifndef EIGEN_DONT_VECTORIZE
   #if defined (EIGEN_SSE2_BUT_NOT_OLD_GCC) || defined(EIGEN_SSE2_ON_MSVC_2008_OR_LATER)
     #define EIGEN_VECTORIZE