Simple to //-ise over number of output classes. Doing the same over
number of trees seems to yield no benefit for ~1000 depth=3 tree so this
was skipped (binary and multi class).

Optimal would be to //-ise over events, but the current design makes
this tricky.

The INCLUDE_DIRECTORIES property only became available in
CMake 3.11, so we have to use COMPILE_FLAGS instead, which
is already available in our required version (CMake 3.4.3).

Note: The ROOT_ARCHITECTURE value of win32old is not found anywhere
else in the code, and the files referred to don't exist anymore, so
the obsolete linkdef file has been removed too.

It is fixing:
..root/tree/dataframe/src/RDataFrame.cxx:789:6: error: reference to 'RDF' is ambiguous
   : RDF::RInterface<RDFDetail::RLoopManager>(
     ^
In module 'ROOTDataFrame' imported ..root/tree/dataframe/src/RDataFrame.cxx:14:
..include/ROOT/RCutFlowReport.hxx:29:11: note: candidate found by name lookup is 'ROOT::RDF'
namespace RDF {
          ^
..root/tree/dataframe/src/RDataFrame.cxx:721:11: note: candidate found by name lookup is 'ROOT::Detail::RDF'
namespace RDF {
          ^
..tree/dataframe/src/RDataFrame.cxx:789:11: error: member initializer (null) does not name a non-static data member or base class
   : RDF::RInterface<RDFDetail::RLoopManager>(
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This reverts commit 80b1a73b.

The patch triggers a pre-existing bug on fedora28 and gcc8.1

This is a better implementation over doing #ifdef R__USE__CXXMODULES
everywhere.