Problem: Android build scripts silently fail (fix make clean)
Problem: Android build scripts silently fail (fix make clean)

Scenario:
```
prompt> cd czmq/builds/android    # (seen also with `cd zyre/build/android`
prompt> ./ci_build.sh
...
```

Fails with:
```
make[2]: Leaving directory `/tmp/android_build/arm/libzmq'
make[1]: Leaving directory `/tmp/android_build/arm/libzmq'
libzmq android build succeeded
make: *** No rule to make target `clean'.  Stop.
```

Analysis:
Failure is `<project>/builds/android/build.sh`:
```
(android_build_verify_so "libczmq.so" "libzmq.so" &> /dev/null) || {
    rm -rf "${cache}/czmq"
    (cp -r ../.. "${cache}/czmq" && cd "${cache}/czmq" \
        && make clean && rm -f configure config.status)
```

Can be understood as: copy current source code and clean it for a next rebuild.

`make clean` fails because sometimes, it is called just ater a `git clone`.
Then, `./autogen.sh` and `./configure` are not yet called to generate the
`Makefile`.

- Solution: use `( make clean || : )`.
Actually, this is the same as already in place in LIBZMQ.
(Problem: Android build script may fail silently)

zeromq/zproject,zeromq/zproject,zeromq/zproject