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A general description is here.
(OS X only.) Package ‘tcltk’ checks when loaded if it is linked against the CRAN X11-based Tcl/Tk and if so that the Tcl/Tk component and the X11 libraries are installed. This allows more informative error messages to be given.
The ‘X11()’ device and X11-based versions of the data editor and viewer (invoked by ‘edit()’ and ‘View()’ for data frames and matrices) check that the X11 libraries are installed.
‘icuSetCollate()’ allows ‘locale = "default"’, and ‘locale = "none"’ to use OS services rather than ICU for collation.
Environment variable ‘R_ICU_LOCALE’ can be used to set the default ICU locale, in case the one derived from the OS locale is inappropriate (this is currently necessary on Windows).
The ‘configure’ script reports on the more important capabilities/options which are not compiled in.