* Fix a compilation issue with large fixed-size matrices: the unrollers were always instanciated.

* the unrolling limits are configurable at compile time.

test/basicstuff.cpp

@@ -89,6 +89,7 @@ void EigenTest::testBasicStuff()
     basicStuff(MatrixXcf(3, 3));
     basicStuff(MatrixXi(8, 12));
     basicStuff(MatrixXcd(20, 20));
+    basicStuff(Matrix<float, 100, 100>());
   }
 }