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A general description is here.
The included BLAS sources have been updated to those shipped with LAPACK version 3.10.1. This causes some (platform-dependent) changes to package check output.
‘USE_FC_LEN_T’ is the default: this uses the correct (compiler-dependent) prototypes for Fortran BLAS/LAPACK routines called from C/C++, and requires adjustment of most such calls - see ‘Writing R Extensions’ §6.6.1.