Avoid interference from stuff left over by a previous arch by performing clean up between builds.

It looks like running a build for one arch with autotools interferes with the next (despite doing a full ./autogen.sh && ./configure). At least the 'first' seems to succeed while the 'second' fails, judging by the build failure of: https://binary-factory.kde.org/job/Keysmith_android/91/execution/node/11/log/

There's always a bigger hammer, though: try to fix it by cleaning up between builds and always starting from scratch with a fresh clone.