generate.sh: "Directory open failure..."

[evoskuil]

gsl/4 M: Directory open failure: '/home/travis/build/libbitcoin/libbitcoin-system' No such file or directory

https://travis-ci.org/github/libbitcoin/libbitcoin-build/builds/763793897#L381

This has also been happening on master:
https://travis-ci.org/github/libbitcoin/libbitcoin-build/builds/761398269#L380

This is working properly via cmd files on Windows using the same generate.xml.

#259 (comment)

[evoskuil]

These errors are apparently resulting from empty directories in the various repos:

gsl/4 M: Directory open failure: '/home/travis/build/libbitcoin/libbitcoin-database/tools/block_db' No such file or directory