Add Suggests: for conditional dependencies

[s-u]

Some dependencies are optional, so they should be declared in Suggests:. This is not important for the builds, but it is important for the new dependency resolution system since we need to know if an optional library has been used during the build. At the time of manifest generation we need to check whether suggested packages are also present and add them to BuiltWith:.

This will solve issues like FreeType's optional dependency on Harfbuzz (see #23)

[s-u]

It is now implemented in a0488a8 such that BuiltWith is conditionally updated. However, build cycles have to be broken by hand, i.e. there is no automated bootstrap + re-build.