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Co-Authored-By: Vassil Vassilev <v.g.vassilev@gmail.com>

[CMake] REFLEX_GENERATE_DICTIONARY: add USES parameter to pass targets from which include directories and definitions can be obtained

This should fix the runtime_cxxmodules on osx.

Move several virtual methods to implementation file

Do not expose X to GUI classes - they should include TVirtualX.h only
when required. 

In ROOT-9516, there is a call to the Roofit::Minimizer function
that returns a RooCmdArg object by value. In such a case, we must
not delete the C++ object in TMemoryRegulator::RecursiveRemove,
since cppyy already does it as part its RecursiveRemove.

Move the creation of `TThreadExecutor pool` before the call of `Internal::MakeClusters()`, to make sure it is properly initialized when calling `ROOT::Internal::TPoolManager::GetPoolSize()` (via `ROOT::GetImplicitMTPoolSize()`)
This fix a crash with the `gtest-tree-treeplayer-test-treeprocessormt` test on Windows, due to a division by zero at line 224 in `TTreeProcessorMT`