This patch allows TFileMerger to work with externally created TFile-s. Being
able to control the creation of the TFile objects give us a chance to use
in-memory files. This is very helpful in benchmarking when we want to simulate
fast disks or we just want to avoid disk wearout.

but rather keep track of that status as a simple boolean in libCore.
libImt is presently the tool used via TROOT to get the status of the activation
of ImplicitMT. That requires the load of libImt and the invocation of a function.
This PR made of 1 commit simplifies the mechanism, avoiding to load libImt (and
initialise the entire TROOT) in case only the activation status of ImplicitMT is
required.

Issue reported in https://sft.its.cern.ch/jira/browse/ROOT-9043

- In SelectionRules.cxx: #define fnmatch (Unix filename pattern matching) as PathMatchSpec (MS-DOS)
- In TModuleGenerator::WriteRegistrationSource, since Visual sudio has a limitation of 2048 characters max in raw strings, split the potentially huge DICTFWDDCLS raw string into multiple smaller ones
- In rootcling_impl.cxx, implement the Windows specific mangled/demangled checks, and convert several backslashs into forward slashs

this is a requirement to allow the TInterface to get rid of move
semantics in the interface and allow PyROOT to work.

Use a EStatusBits 'enum' instead of a 'constexpr static int' (allowing automatic checking for overlaps)

Following the observation from philippe: Macro named starting with underscore are technically reserved for/by the compiler

- Add a couple of missing #include <algorithm>
- Replace _N by _NN (_N is already defined and used in some system math headers on Windows)
- Add #define __thread __declspec(thread)