As required under the LGPL, a prominent notice has been added to the
main readme file to inform the user that this is a modified version
of ROOT.

This adds additional directories to the CMake variable
CLING_CXX_HEADERS, whose content is later on compiled into
the ROOT libraries.

Cling requires access to the header files even at runtime,
but a Flatpak runtime environment does not contain them,
only the SDK used at compilation time does.

The /usr tree inside a Flatpak sanbox is immutable, everything
the application needs in addition to the underlying runtime has
to be shipped in /app.

This patch adds these paths under /app to the search paths of Cling
so that it does work inside a Flatpak sandbox.

This behaviour is controlled by the new boolean build option
“BUILD_FOR_FLATPAK”.

However, this patch alone does not do the trick. The Flatpak build manifest
needs to contain commands to create the “/app/include” directory and copy
all the include files there from the SDK as well as to install the include
files from ROOT under “/app/include/root”.

This compiler flag — foreseen by the original authors — stops RooFit
from displaying its banner in front of the banner and license
information of the main application

Remove "l" from "ar clq" command while there is no extra library
linked to libAfterImage.a

Otherwise image slowly degrade if doing loop:
for (int i=0;i<100;++i) {
   img1->GetImageBuffer(&buf, &sz);
   img1->SetImageBuffer(&buf);
}

With latest libpng versions value 0 recognized as error and
produced results are wrong

If application runs in special thread, one should
call RWebWindowManager::AssignMainThrd() to indicate this

This enum will be removed shortly (functionality moved from TObject::Write to *::Merge)

This is necessary to support the 'fast' incremental merge where the input will continue to be used.

TIsAProxy:  Don't record failed searches (a library might be loaded between now and the next search).

Since fClass is modified only 'once' at init time of the IsAProxy object, it is actual more efficient
to protect the initialization by a lock rather than having fClass being atomic.

This allows to significant reduce the number of search through the list of cached result ...
That search was a noticeable cause of slow down with an increase of number threads due to the contention
on updating the number of readers (used for the local read/write lock mechanism).

To save space, the compression can be re-enabled by TBuffferMerger::SetCompressTemporaryKeys.

The TBaskets are still compressed and thus the end result is unchanged.

TClass::GetBaseClassOffset is a 'often' used routine in the I/O.
In the fast path (looking up in a cache), rather than taking the
global read lock (which is not only global but also 'slower' than
a regular mutex), we are now use a instance specific mutex to
protect the cache.

Add new enum for TObject::Write: kOnlyPrepStep  = BIT(3)         ///< Just prepare the objects do not write them (i.e. call TTree::FlushBaskets)

If TBufferMerger can take the merge lock, then rather than queuing the input file, merge it immediately
and skip the Writing the TTree, doing a memcpy, Pushing to the queue then Reading and then deleting parts.

Currently only in the case where we have a Merge function and a dictionary.

For iteration over the list of baskets, the existing code try to reduce the range of the iteration
to stop as soon as all non-nullptr slot have been seen.
However to know the number of non-nullptr slot, the code (has to) call GetEntries which scan the
entire array anyway ...

TObjArray::GetEntries() counts the number of non-zero slots
while TObjArray::GetEntriesFast() return (in most case) the value of a high water mark (the slot over which all remaining slots
are empty).

In one use case (doing read a TTree and do many TTree cloning (via TBufferMerger), TObjArray::GetEntries was taking 8% of the
running time (compression was disabled)

Note: TObjArray::IsEmpty use GetEntriesFast rather than GetEntries.

Limited to TTree and Histogram.

We do not apply the same to CloneTree as the addresses copying
is a side-effect (no reset at the end) that is relied upon by
user code.g

This can be used to monitor the queue size in bytes rather than number of files

This can now be safely use to avoid overloading the queue.