Now that the build directory is found automatically, if it so
happens that you have more than one build dir in your tree,
'make run' may not run the version that you expect...
./builddir/wp-uninstalled.sh can now be called without a -b argument,
it'll automatically set the full paths. This is useful when building as
a subproject.
meson has a limitation: the configure_file() object is just a file
wrapper object. Wrap this into a custom_target() whose return value has
a full_path() method. This way, pipewire can use
wireplumber_uninstalled.full_path() without having to build the
directory structure itself.
