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A general description is here.
An alternative compiler for C++11 is now specified with ‘CXX11’, not ‘CXX1X’. Likewise C++11 flags are specified with ‘CXX11FLAGS’ and the standard (e.g., ‘-std=gnu++11’ is specified with ‘CXX11STD’. The variables with prefix ‘CXX1X’ are defunct.
‘configure’ now tests for a C++14-compliant compiler by testing some basic features. This by default tries flags for the compiler specified by ‘CXX11’, but an alternative compiler, options and standard can be specified by variables ‘CXX14’, ‘CXX14FLAGS’ and ‘CXX14STD’ (e.g., ‘-std=gnu++14’).
Packages which register native routines for ‘.C’ or ‘.Fortran’ need to be re-installed for this version (unless installed with R-devel SVN revision r72375 or later).