Fix VPATH build

Fix VPATH build

When trying to build shadow in a different directory I stumbled upon few
issues, this commit aims to fix all of them:

- The `subid.h` file is generated and hence in the build directory and
	not in the source directory, so use `$(builddir)` instead of
	`$(srcdir)`.

- Using `$<` instead of filenames utilises autotools to locate the files
  in either the source or build directory automatically.

- `xsltproc` needs to access the files in login.defs.d in either the
  source directory or the symlink in a language subdirectory, but it
	does not interpret the `--path` as prefix of the entity path, but
	rather a path under which to locate the basename of the entity
	from the XML file.  So specify the whole path to login.defs.d.

- The above point could be used to make the symlinks of login.defs.d
  and entity path specifications in the XMLs obsolete, but I trying
	not to propose possibly disrupting patches, so for the sake of
	simplicity just specify `$(srcdir)` when creating the symlink.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>