- Remove the `.so` suffix and any trailing whitespace in the output generated
by `libs.C`.

- Remove dead code in `libs.C`

`modifyClassWebpages.sh` might spawn many `modifyClassWebpage.sh` processes
via `xargs -P`. In turn, `modifyClassWebpage.sh` depends on `libs.C`
which is ACLiC'ed if to generate `libs_C.so` where required.

However, if `libs_C.so` does not exist, multiple processes might race to
create it. This commit ensures that the macro is ACLiC'ed before
`modifyClassWebpage.sh` runs.

- fix `mktemp` command lines to only use template 'X's at the end

- use `sysctl -n hw.ncpu` instead to get the number of available CPUs on Darwin

- move OS-dependent variable assignments to the Makefile

- Doxygen was creating one svg for every class, showing in which library it resides. This created
  6000 svgs, which were mostly identical. Now, only ~100 are created for every library in ROOTSYS/lib/,
  and the relevant webpages link to those.
  This speeds up the "listlibs" step of the doxygen process signficantly.
- Add parallel processing for collaboration diagrams of class webpages.
- Remove scanning of rootmap files. It doesn't work, any more. TCling needs
  a fix for ROOT-10900, so gInterpreter->GetClassSharedLib() will always
  return the right thing.
- Further, parallelise the modification of class webpages.

[ROOT-10157]
- The doxygen filter step now only writes out that it wants a
notebook, and how it should be created, but it doesn't run the notebook.
- Later, this info is picked up by makeNotebooks.sh, and notebooks are
generated in parallel.
- Add a variable in the Makefile to choose number of processes for notebook
  conversion.

converttonotebook.py was converting each comment into a markdown cell.
If comments are inside blocks, the blocks get split into multiple cells,
and therefore cannot execute, any more.
[ROOT-10877]

If errors/exceptions occur during conversion of tutorial macros to
notebooks, a few more helpful messages are printed. Further, the exit
status of the notebook conversion step now propagates to the outside,
the conversion failures can be detected automatically.

- Add a function that measures the indentation of a function body.
- Use this to create nicely formatted cells in notebooks. This is
especially important for RooFit tutorials.

converttonotebook.py was able to recognise a few functions based on a
few hard-coded return types in C++. Now, it searches for patterns such as
   <return type>[*&] [abcd::]abcd(
to find functions, and then it counts () as well as { } to find the
function bodies. It also more reliably deals with things that appear
before function definitions, such as
   template<typename T>

[ROOT-10861] This is a partial fix for ROOT-10861, because functions are
not ripped apart, so the notebook will run.

Further, doxygen-style latex tags such as `\f[` by
notebook-style `$$`, so markdown cells look better.

Make sure converttonotebook recognises indented comments.

* [DF][NFC] Mention change in behavior of Alias in rel notes

* [skip-ci] make sur the http doc is read by doxygen.

Co-authored-by: Enrico Guiraud <enrico.guiraud@cern.ch>

Copy from JSROOT repository

* Fix DF examples to make them work as notebooks

* Add missing space

* python exec in filter.cxx

* use "authors"

* notebook does not make sense for these tutorials.

Since JSROOT v6 all functions names are "onWebsocketMsg"

* precompile

* add missing headers

* MetaSema: add short version of help, as in quit
Primer: join help and ?, mention .quit and .exit
TRint: mention .?
TApplication: mention .? and CINT 2 Cling

* clarify class name with ROOT7

https://docs.mathjax.org/en/latest/upgrading/whats-new-3.0.html#improved-speed

Also the size of the tar file is smaller

and avoid corresponding warnings.
This follows the recommendation from https://www.doxygen.nl/manual/preprocessing.html concerning hiding attribute and declspec from doxygen.

* Check non-NULL result of fopen, to prevent crash

In the Doxyfile, when setting NUM_PROC_THREADS to 8 so that the documentation builds faster, sometimes fopen fails and returns NULL, and after that the process crashes. This guard prevents that situation.

* more safeguards

* [doxy] add some more folders to include path

To prevent many doxygen warnings regarding 'Streamer' function.

* use a different virtual folder

to prevent name conflicts with other help books

* avoid doxygen warning

needed to avoid warning about clang/Basic/LangOptions.def not being found

* avoid another warning

from RAttrAxis

* and another for tmva register_method calls

Solves hundreds of doxygen warning in Streamer class.

The pythonizaiton of some RooFit global functions merited their own
documentations, which are added in this commit.

To make these documentations accessible from doxygen, the following
changes were made in addition:

  * the `print_roofit_pyz_doctrings.py` script now also considers free
    functions

  * the global functions are correctly linked to in the documentation of
    `RooAbsData::plotOn`