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A general description is here.
There is now support for installing the debug symbols for recommended packages on macOS: see ‘REC_INSTALL_OPT’ in file ‘config.site’.
Strict R headers are now the default. This removes the legacy definitions of ‘PI’, ‘Calloc’, ‘Realloc’ and ‘Free’: use ‘M_PI’, ‘R_Calloc’, ‘R_Realloc’ or ‘R_Free’ instead.
‘R CMD check’ with a true value for environment variable ‘_R_CHECK_BASHISMS_’ checks more thoroughly, including for ‘bash’ scripts and components of ‘autoconf’-generated ‘configure’ scripts.