Add yaml-cpp and boost

This diff adds yaml-cpp and all the boost submodules that
are needed to compile yaml-cpp.

We initially tried with cmake, but it was not straightforward
to interface cmake and GNU autotools.

Therefore, this diff implements a different approach by
which we generate an include.am for yaml-cpp.
Teach ./configure about yaml-cpp and propagate changes
configure.ac: square brackets are handled specially by m4

So, let's use `test ...;` rather than `[ ... ];`.
configure.ac: correctly include boost headers
configure.ac: fix: save/restore CCPFLAGS not CXXFLAGS
configure.ac: fix: $ight_yamlcpp_path default value is "" not "check"
configure.ac: fix: LDFLAGS += -lyaml-cpp in any case
configure.ac: fix: run all yaml-cpp checks in C++ mode
configure.ac: explain common reasons why yaml.h cannot be used
configure.ac: use --with-libyaml-cpp not --with-yaml-cpp
Teach configure.ac about libboost

We only check for the bare subset of boost that we need to
compile the yaml-cpp library version 0.5.1.
configure.ac: fix boost description
Merge pull request #38 from bassosimone/feature/add_yaml_cpp-try3+autoconf

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