libopencash: support tests on Android

An issue remains where adding a POCO delegate to an event deadlocks the
thread. This means that any test using {Mock}ModelObserver will hang.
The cause is still unclear, but I've tracked it down to
pthread_mutex_lock blocking. That should not happen since FastMutex will
always work on different mutex objects, but who knows...
libopencash: fix compilation and tests for Android

The following changes were made:
- ./build.sh now creates tests by default, except for platforms
  ios-device and ios-simulator
- reverted poco back to version 1.5.2 and added patches for clang-atomic
  and strerror_r