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A general description is here.
‘R CMD config’ no longer knows about the unused settings ‘F77’ and ‘FCPIFCPLAGS’, nor ‘CXX98’ and similar.
Support for specifying C++98 in package installation is deprecated.
‘installTrChar’ (which is nowadays is wrapped by ‘installChar’) is defined in ‘Rinternals.h’. (Neither are part of the API.)