Basically it seems that in the command:
"cp  $W/Extension/report-builder/root"
 it's missing the first operand....
It is a use case that nobody has ever tried before, because en-US is the
default language. The line comes from solenv/gbuild/ExtensionTarget.mk
and the missing argument(s) is a list of files based on $(gb_WITH_LANG),
but _without_ en-US. Since $(gb_WITH_LANG) contains _nothing but_ en-US
in your case, there are no files to copy...
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