- 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.

test/product.cpp

@@ -100,7 +100,7 @@ void EigenTest::testProduct()
   }
 
   // test a large matrix only once
-  product(Matrix<float, 100, 100>());
+  product(MatrixXf(100,100));
 }
 
 } // namespace Eigen