Most dependencies are libraries. Almost all build systems can find dependency libraries from the system using e.g. pkg-config. Some can build dependencies from source. Some, like Meson, can do both and toggle between them transparently. Library dependencies might not be a fully solved problem but we as a community have a fairly good grasp on how to make them work.
However there are some dependencies where this is not enough. A fairly common case is to have a dependency that has some sort of a source code generator. Examples of this include Protocol Buffers, Qt's moc and glib-mkenums and other tools that come with Glib. The common solution is to look up these binaries from PATH. This works for dependencies that are already installed on the system but fails quite badly when the dependencies are built as subprojects. Bootstrapping is also a bit trickier because you may need to write custom code in the project that provides the executables.
Version 0.46.0 which shipped yesterday has new functionality that makes this use case noticeably simpler. In Meson you find the code generator scripts to run with the find_program command like this:
mkenums_exe = find_program('glib-mkenums')
This will find the executable from the system. However if you have built Glib as a subproject, then it can issue the following statements (this is not in Glib master yet AFAIK so it does not work, this is more of an illustrative example):
internal_mkenums_exe = <command to generate the mkenum script>
After this issuing find_program('glib-mkenums') no longer goes to the system, but instead returns the internal program. Meson's internal helper modules have also been updated to always find the programs they use with find_program. This means that all projects using Glib functionality can be built without needing a system wide install of Glib. Even more importantly this requires zero changes in existing projects. It will just work out of the box. You can even use Glib helper code when building Glib itself.
This is especially convenient when you need a newer version of any dependency than your distro provides and especially on platforms such as Windows where "distro dependencies" do not exist.
As an example of what is possible, Nirbheek has managed to bootstrap GStreamer on Windows using nothing but Visual Studio, Python 3, Ninja and Meson. The main limitation currently is that the overriding executable may not be a build target (i.e. something you build from source with a compiler) because the result of find_program may be used during the configuration phase, before any source code compilation has taken place. We hope to remove this limitation in a future release.