CMake build: Allow exclusion of Boost.Asio sources from the build.

By passing "-D asio_source=OFF" to cmake, it's possible to build
the Asio compilation unit in a parent project, which makes it
easier to combine libtorrent with other libraries that also depend
on Asio.
CMake build: Allow building with C++11 support.
CMake build: Automatic clang color diagnostics when building with Ninja.