Add BuildSources= match

Add BuildSources= match

One pattern I've started using a lot when I have to build multiple
projects from source in a build script is to check if a source tree
has been mounted at some location using BuildSources= and to only
build the project if that's the case.

The problem with this is that this only allows me to skip the build,
it doesn't allow me to skip installing the necessary build and runtime
packages for that particular project.

Let's add a BuildSources= match so that everything related to the
project can be skipped if the project is not configured to be mounted
at some location using BuildSources=.