Improve prep_buildtree

Since I started using meson regularly, I have put the build directory
under the source directory, which appears to be the recommended
convention. See for example

https://mesonbuild.com/Quick-guide.html#compiling-a-meson-project

Then, if I also want to create a build directory using configure/make in
parallel to that (for example, to test the makefiles, if adding new
source files), then prep_buildtree is a bit stupid about that. It
processes all directories under the top-level source directory,
including existing other build trees, and so it makes a copy of existing
build trees under the new build tree. This isn't actually harmful, but
it seems pretty stupid, and it also makes prep_buildtree slower and
slower the more build trees you have.

I have never been much of a vpath user before meson, so I don't have
much experience with it, but I suspect that it was previously the
convention to have the build tree outside of the source tree, in which
case this wouldn't have been a problem.

To fix this, I first tried to devise a way to detect whether a given
directory is a build directory. But that seemed pretty complicated.
Instead, I chose the simpler solution that we just enumerate the source
subdirectories that we know about (config, contrib, doc, src). That way
we can also remove the special handling to exclude the .git directory
and make the find command a bit simpler.

See the attached patch.

Maybe people can try this with their local workflows to make sure it
still works for them, but I don't expect any particular problems, other
than perhaps some shell or find command portability issues. Note, this
only runs if you do a vpath configure/make build, not an in-tree
configure/make build, and of course not if you use meson.