Merge branch 'build-script' into docs-mw

Quoth Mauro:

This series should probably be called:

"Move the trick-or-treat build hacks accumulated over time
into a single place and document them."

as this reflects its main goal. As such:

- it places the jobserver logic on a library;
- it removes sphinx/parallel-wrapper.sh;
- the code now properly implements a jobserver-aware logic
to do the parallelism when called via GNU make, failing back to
"-j" when there's no jobserver;
- converts check-variable-fonts.sh to Python and uses it via
function call;
- drops an extra script to generate man pages, adding a makefile
target for it;
- ensures that return code is 0 when PDF successfully builds;
- about half of the script is comments and documentation.

I tried to do my best to document all tricks that are inside the
script. This way, the docs build steps is now documented.

It should be noticed that it is out of the scope of this series
to change the implementation. Surely the process can be improved,
but first let's consolidate and document everything on a single
place.

Such script was written in a way that it can be called either
directly or via a Makefile. Running outside Makefile is
interesting specially when debug is needed. The command line
interface replaces the need of having lots of env vars before
calling sphinx-build:

$ ./tools/docs/sphinx-build-wrapper --help
usage: sphinx-build-wrapper [-h]
[--sphinxdirs SPHINXDIRS [SPHINXDIRS ...]] [--conf CONF]
[--builddir BUILDDIR] [--theme THEME] [--css CSS] [--paper {,a4,letter}] [-v]
[-j JOBS] [-i] [-V [VENV]]
{cleandocs,linkcheckdocs,htmldocs,epubdocs,texinfodocs,infodocs,mandocs,latexdocs,pdfdocs,xmldocs}

Kernel documentation builder

positional arguments:
{cleandocs,linkcheckdocs,htmldocs,epubdocs,texinfodocs,infodocs,mandocs,latexdocs,pdfdocs,xmldocs}
Documentation target to build

options:
-h, --help show this help message and exit
--sphinxdirs SPHINXDIRS [SPHINXDIRS ...]
Specific directories to build
--conf CONF Sphinx configuration file
--builddir BUILDDIR Sphinx configuration file
--theme THEME Sphinx theme to use
--css CSS Custom CSS file for HTML/EPUB
--paper {,a4,letter} Paper size for LaTeX/PDF output
-v, --verbose place build in verbose mode
-j, --jobs JOBS Sets number of jobs to use with sphinx-build
-i, --interactive Change latex default to run in interactive mode
-V, --venv [VENV] If used, run Sphinx from a venv dir (default dir: sphinx_latest)

the only mandatory argument is the target, which is identical with
"make" targets.

The call inside Makefile doesn't use the last four arguments. They're
there to help identifying problems at the build:

-v makes the output verbose;
-j helps to test parallelism;
-i runs latexmk in interactive mode, allowing to debug PDF
build issues;
-V is useful when testing it with different venvs.

When used with GNU make (or some other make which implements jobserver),
a call like:

make -j <targets> htmldocs

will make the wrapper to automatically use POSIX jobserver to claim
the number of available job slots, calling sphinx-build with a
"-j" parameter reflecting it. ON such case, the default can be
overriden via SPHINXDIRS argument.

Visiable changes when compared with the old behavior:

When V=0, the only visible difference is that:
- pdfdocs target now returns 0 on success, 1 on failures.
This addresses an issue over the current process where we
it always return success even on failures;
- it will now print the name of PDF files that failed to build,
if any.

In verbose mode, sphinx-build-wrapper and sphinx-build command lines
are now displayed.