Makefile: Streamline directory creation

Some directories don't exist in the repository, but are required at build time
(e.g. doc/html). Until now some were created explicitly, some through the
target “stamp-directories” and other target simply relied on a previous target
to create the directory.

This patch tries to clean this up by getting rid of “stamp-directories” and
instead use rules to recreate any missing directory. As described in a comment
in the code, a file inside each directory is necessary, named “.dir”.

Order-only dependencies are used for directory creation to avoid rebuilding
where only the “.dir” file is missing (see “info make”, section “4.3 Types of
Prerequisites”).

The target for building the documentation is also changed to use “…/index.html”
instead of a hidden file. Some style changes are also made.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>