yaml-cpp: add --with-shared-lib option

The `--with-shared-lib` option allows the user to build a dynamic instead of the default static library. As detailed on the [project's HP](https://code.google.com/p/yaml-cpp/#How_to_Build), the option adds `-DBUILD_SHARED_LIBS=ON` to the cmake command.
This syntax (multiple splats) isn't supported on Ruby 1.8; instead do `args = std_cmake_args` above and then only `*args` here.