currently default_src_install is carried out in $BUILD_DIR and not in $S

that means people have to do something like this:
DOCS=( "${S}"/ChangeLog{,.libffi,.libgcj,.v1} "${S}"/README )

The attached patch is a bit ugly, but I don't see a better way. It will
allow:
DOCS=( ChangeLog{,.libffi,.libgcj,.v1} README )