Make installing non-existing subdirs a supported feature

install_subdir() with a non-existing subdir creates the directory in the
target directory. This seems like an implementation detail but is quite useful
to create new directories for e.g. configuration or plugins in the installed
locations.

git bisect says this started with 8fe816101467e66792251b4f57e0ddddb537764a.
Let's add a test for it and document it to make this behavior official.

Limitation: it can only create at the install_dir location, trying to create
nested subdirectories does not work and indeed creates the wrong directory
structure. That is a bug that should be fixed separately:

        install_subdir('blah',
                        install_dir: get_option('prefix'))
        install_subdir('sub/foobar',
                        install_dir: get_option('prefix'))
        install_subdir('foo/baz',
                        install_dir: get_option('prefix'))

        $ tree ../_inst
        ../_inst
        ├── baz
        ├── blah
        └── foobar

Fixes #2904

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