CMake: Only enable HAVE_TEUCHOS_LINK if BFD=ON

By default, WITH_BFD=OFF, so in this case we need to disable HAVE_TEUCHOS_LINK
as well. Only when BFD is enabled, we need to enable it.

Fixes #152.
Add a test for link.h if WITH_BFD=ON

Now CMake fails if link.h is not present and BFD is enabled.
Merge pull request #161 from certik/link_h

Fix link.h issues