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A general description is here.
The libraries searched for by ‘--with-blas’ (without a value) now include BLIS (after OpenBLAS but before ATLAS). And on macOS, the Accelerate framework (after ATLAS). (This is patterned after the ‘AX_BLAS’ macro from the Autoconf Archive.)
The search for a CRAN mirror when checking packages now uses ‘getOption("repos")’ if that specifies a CRAN mirror, even when it does not also specify all three Bioconductor repositiotries (as was previously required).