Move declaration of ESockOptions and ESendRecvOptions enums from TSocket.h to TSystem.h, to get rid of this include dependencies in CMake:
include_directories(${CMAKE_SOURCE_DIR}/net/net/inc)

in order to allow to call ROOT::EnableImplicitMT() without including
that header.

- Avoid installation of empty lib/pkgconfig directory
- Use PCRE_PCRE_LIBRARY and PCRE_PCREPOSIX_LIBRARY variables

Fixes: ROOT-9864

Do not expand the parent directory tree on network file systems. Thanks to Oliver Freyermuth for the report and the list of file systems

The method TClass::GetClass<T> obtains the TClass instance pointer
corresponding to T in two ways, according to the nature of T.
If T inherits from or is TObject, the TClass* value is obtained via the
T::Class() static method, which locks only upon the first call.
If T is a generic type, internally the TClass::GetClass(type_info, ...)
method is called.

The new functionality is put to a good use by this commit in TBuffer
and TDirectoryFile.

It is worth noting how the usage of tag dispatching could be replaced
by 'if constexpr'.

in PyROOT, it is not possible to iterate on a RResultPtr<T> as if it was a simple
T. The workaround consists in getting the value of the result pointer explicitly.

A number of test failures have to be fixed in the experimental
PyROOT builds. This commit temporarily flags those failing
tests as "will fail" for the experimental PyROOT builds.
They will be restored progressively as they are fixed.

This reverts commit fb2bea8c.
Our FindGSL.cmake is not equivalent to the version distributed
with CMake, and our build system needs to use our version to
work. Both Windows builds with external GSL and the Travis CI
build seem to be affected by this.

this was possible given that now the invocation of templated methods
is supported.
The code of the tests was also improved.

revert until the problem with capturing are well understod.

This reverts commit 241fc7f0.

When several templated overloads exist, we only only need to inject
the corresponding TemplateProxy once.

This will allow all the method overloads to be added later to the
TemplateProxy that was previously injected.

When building the dictionary of a PyROOT class proxy, the member
function templates need to be added too as TemplateProxies.
Otherwise, they are not found when the user tries to instantiate
the template.

Any non-templated overload will be added to the TemplateProxy, so
that the user can invoke those as well through the latter.

Since TDirectoryFile redefines GetObject, we need to re-inject its
pythonization (which is added to TDirectory too). Otherwise, invoking
GetObject on TDirectoryFile will not access the pythonization when
a template proxy is added to TDirectoryFile for GetObject.

and therewith notebooks.
This was necessary to work around fakemodule, which has been removed
from ipython a long time ago.
The invocation to TPython triggers the loading of the TPyClassGenerator
TClassGenerator. Such generator acquires the GIL from within C++ code
unavoidably leading to deadlocks if ROOT thread safety is enabled.

but rather use a single thread dedicated to the async execution.

This reverts commit f84668d7.
It leads to test failures on centos7, while all other systems are ok.

Single and multi-threaded.

- When RooWorkspaces that contain a ModelConfig are cloned, the ModelConfig did not get updated. It would still point to the old workspace.
- Add unit test.

This was introduced by 2744a802520eb0f45a832ed2c9d351c30b73cee9.
Currently, 'cmake -E env' is used for setting environment variables.