Build break - armel - invalid use of incomplete type 'class std::future<void>

[bobsummerwill]

[ 75%] /home/crosseth/eth/wd/cpp-ethereum/test/libethereum/test/TestHelper.cpp: In function 'void dev::test::testMiningFunc(void (*)())':
/home/crosseth/eth/wd/cpp-ethereum/test/libethereum/test/TestHelper.cpp:1101:79: error: invalid use of incomplete type 'class std::future<void>'
   auto future = std::async(std::launch::async, &std::thread::join, &threadTest);
                                                                               ^

• https://s3.amazonaws.com/archive.travis-ci.org/jobs/155485648/log.txt (armel)
• https://s3.amazonaws.com/archive.travis-ci.org/jobs/155485649/log.txt (armhf)
• https://s3.amazonaws.com/archive.travis-ci.org/jobs/155485650/log.txt (armel-apt)

Looks like it's an issue with the use of std::future on this particular architecture.

This is brand new code from @winsvega for adding the dots to keep TravisCI alive, which is obviously constraining portability. We should probably do something simpler with std::thread instead.

This particular error log is for the armel cross-build (armv5) but it looks like there are probably more.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64735

[bobsummerwill]

One of the two instances of this has been removed upstream (mining timeouts).

The other (the dots) remains for now, but we'll look at this too.

[bobsummerwill]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64735

Was fixed in 2017, so this might not be an issue anymore, if the GCC within the Docker image was updated accordingly.