Compilation error when using clang

Where couple of issues:

- Overloaded assignment operator usage ambiguity
  around some of the Eigen matrix assignment.

- Using -O4 flag here on linux ended up in lots
  of cryptic linker issues, even when using recently
  release clang 3.3.

  Disabled forcing optimization flag for now.
  We could end up with something smarter in the
  future, but for now i'm not as much fan of
  forcing compiler's flag.