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A general description is here.
Header ‘Rconfig.h’ contains the value of ‘FC_LEN_T’ deduced at installation which is used by the prototypes in headers ‘BLAS.h’ and ‘Lapack.h’ but to avoid extensive breakage this is only exposed when ‘USE_FC_LEN_T’ is defined.
If a package's C/C++ calls to BLAS/LAPACK allow for the ‘hidden’ arguments use by most Fortran compilers to pass the lengths of Fortran character arguments, define ‘USE_FC_LEN_T’ and include ‘Rconfig.h’ (possibly _via_ ‘R.h’) before including ‘BLAS.h’ or ‘Lapack.h’.
‘R CMD config’ knows the values of ‘AR’ and ‘RANLIB’, often set for LTO builds.