build/doc: Ensure destination dir exists despite VPATH

Make considers a variable called VPATH when trying to satisfy
dependencies, e.g. for a target 'foo', it will consider the target
extant if VPATH is '../../bar' and '../../bar/foo' exists.

Part of the doc build, the '$(alldirs)' target, exists to create the
target directories if they do not exist. For example, before generating
xml/wayland-architecture.png, it will ensure the 'xml' target is
considered up-to-date thanks to the target dependency.

Creating $(srcdir)/doc/doxygen/xml thus means that the 'xml' dependency
will be satisfied, so we'll never create the output directory, and the
doc build will fail.

Change the alldirs target list to be absolute paths, so VPATH will not
be consulted and defeat the entire point of what we're trying to do.
This fixes the Meson build, where we later create
doc/doxygen/xml/meson.build.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>