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A general description is here.
‘configure’ option ‘--with-lapack=no’ (equivalently ‘--without-lapack’) forces compilation of the internal LAPACK sources.
If ‘--with-lapack’ is not specified, a system ‘liblapack’ is looked for and used if it reports version 3.10.0 or later.
Using 3.10.0 may give some different signs in SVDs (seen in the ‘lapack.R’ test).
Packages using LAPACK will need to be reinstalled if this changes to using an external library.
[In progress: scheduled for May 5 if enough packages are corrrected before then.
Setting environment variable ‘_R_INSTALL_USE_FC_LEN_T_’ to a true value enables this _pro tem_. This is set by ‘R CMD check --as-cran’ when it installs a package.]