- cleaner use of OpenMP (no code duplication anymore)

using a macro and _Pragma.
- use OpenMP also in cacheOptimalProduct and in the
  vectorized paths as well
- kill the vector assignment unroller. implement in
  operator= the logic for assigning a row-vector in
  a col-vector.
- CMakeLists support for building tests/examples
  with -fopenmp and/or -msse2
- updates in bench/, especially replace identity()
  by ones() which prevents underflows from perturbing
  bench results.

bench/benchmark.cpp

@@ -18,12 +18,11 @@ USING_PART_OF_NAMESPACE_EIGEN
 
 int main(int argc, char *argv[])
 {
-    Matrix<SCALAR,MATSIZE,MATSIZE> I;
+    Matrix<SCALAR,MATSIZE,MATSIZE> I = Matrix<SCALAR,MATSIZE,MATSIZE>::ones();
     Matrix<SCALAR,MATSIZE,MATSIZE> m;
     for(int i = 0; i < MATSIZE; i++)
         for(int j = 0; j < MATSIZE; j++)
         {
-            I(i,j) = (i==j);
             m(i,j) = (i+MATSIZE*j);
         }
     asm("#begin");