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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 propotypes 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’.
The default detection of the shell variable ‘libNN’ is overridden for derivatives of Debian Linux, some of which have started to have a ‘/usr/lib64’ directory. (E.g. Ubuntu 19.04.) As before, it can be specified in ‘config.site’.