Add yaml-cpp and the set of boost libs required to compile it

This pull request contains 8b5aca9 that adds yaml-cpp and the set of boost libaries required to compile it.

Yaml-cpp is compiled using an `include.am` generated for this purpose, because the interaction of cmake and autotools was taking too much time to converge.

On top of 8b5aca9 there are other patches that teach our autotools infrastructure about yaml-cpp and boost.

As done for libevent2, yaml-cpp is only compiled if the user requests that (i.e., using `--with-libyaml-cpp=builtin`) or if yaml-cpp is not found on the system directories.

Regarding testing, I tested all of this in the following configurations:

1. a barebone Debian box in which dependencies (i.e., libevent2, boost and yaml-cpp) were not installed;

2. a Xubuntu box in which dependencies were installed at `/usr`;

3. a FreeBSD box in which I tested the case in which dependencies were not installed; the case in which dependencies were installed at `/usr/local`; the case in which dependencies were installed in different directories below `/opt`.

In cases 1 and 2, no `./configure` command line arguments were needed. In case 3 the `--with-foo` command line argument were passed to `./configure` when the dependencies were installed.