SSIA. See also Bug 134727
I don't agree with this bug, as it assumes that JHBuild "comes first" and then come the system packages when, in my mind at least, the system packages form the "base" for building WebKit and the Jhbuild system complements that for systems where those packages are not recent enough, or simply to help "normalize" environments for other purposes, such as testing For more details on my reasoning, please look at https://bugs.webkit.org/show_bug.cgi?id=134727#c10