Add config option to build documentation as a two-step process

Add config option to build documentation as a two-step process

The new 'prepare_docs' CONFIG option triggers the documentation rules in
default_post to generate two extra targets: prepare_docs and generate_docs.
The prepare_docs stage runs qdoc with the -prepare option, which means qdoc
will only generate index files, and the generate_docs stage will call
qdoc with -generate, which reads the index files and generates the final
output. The regular docs target will then run the prepare_docs target
for all submodules before running the generate_docs target.

This ensures that when generating the final output, qdoc has all the
index files for all the other modules available, to be able to resolve
cross-references between the various Qt modules.

This patch needs a follow-up in qt5.git to add CONFIG+=prepare_docs, so
that the root Qt5 build will be able to hook into this new behavior.

Change-Id: I654d7f0d4d5a41d9be208e6d3a8923bf0194f9ad
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>