Compiling error on Ubuntu 16.04
Hi @moneroexamples , thanks for your awesome project.

I was able to build Monero following the [instructions](https://github.com/moneroexamples/monero-compilation/blob/master/README.md#example-compilation-of-master-branch-ie-development-version-of-monero-on-ubuntu-1804), but the rest steps to build OpenMonero failed. I got the following errors:

```sh
root@a2b653fe37f6:~/openmonero/build# cmake ..
-- The C compiler identification is GNU 5.4.0
-- The CXX compiler identification is GNU 5.4.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Found MySQL++: /usr/include/mysql++;/usr/include/mysql, /usr/lib/libmysqlpp.so;/usr/lib/x86_64-linux-gnu/libmysqlclient.so
-- Found Restbed include at:
    /root/openmonero/ext/restbed
-- Configuring directory /root/openmonero/build/config
-- Configuring file config.json
-- MONERO_DIR: ~/monero
-- FindMonero Xmr_CRYPTONOTE_CORE_LIBRARIES /root/monero/build/release/src/cryptonote_core/libcryptonote_core.a
-- FindMonero Xmr_BLOCKCHAIN_DB_LIBRARIES /root/monero/build/release/src/blockchain_db/libblockchain_db.a
-- FindMonero Xmr_CRYPTONOTE_BASIC_LIBRARIES /root/monero/build/release/src/cryptonote_basic/libcryptonote_basic.a
-- FindMonero Xmr_BLOCKS_LIBRARIES /root/monero/build/release/src/blocks/libblocks.a
-- FindMonero Xmr_LMDB_LIBRARIES /root/monero/build/release/external/db_drivers/liblmdb/liblmdb.a
-- FindMonero Xmr_RINGCT_LIBRARIES /root/monero/build/release/src/ringct/libringct.a
-- FindMonero Xmr_RINGCT_BASIC_LIBRARIES /root/monero/build/release/src/ringct/libringct_basic.a
-- FindMonero Xmr_COMMON_LIBRARIES /root/monero/build/release/src/common/libcommon.a
-- FindMonero Xmr_EASYLOGGING_LIBRARIES /root/monero/build/release/external/easylogging++/libeasylogging.a
-- FindMonero Xmr_DEVICE_LIBRARIES /root/monero/build/release/src/device/libdevice.a
-- FindMonero Xmr_EPEE_LIBRARIES /root/monero/build/release/contrib/epee/src/libepee.a
-- FindMonero Xmr_CHECKPOINTS_LIBRARIES /root/monero/build/release/src/checkpoints/libcheckpoints.a
-- FindMonero Xmr_VERSION_LIBRARIES /root/monero/build/release/src/libversion.a
-- FindMonero Xmr_CNCRYPTO_LIBRARIES /root/monero/build/release/src/crypto/libcncrypto.a
FOUND Monero_LIBRARIES: cryptonote_core;blockchain_db;cryptonote_basic;blocks;lmdb;ringct;ringct_basic;common;easylogging;device;epee;checkpoints;version;cncrypto
-- /root/monero/build
-- Looking for pthread.h
-- Looking for pthread.h - found
-- Looking for pthread_create
-- Looking for pthread_create - not found
-- Looking for pthread_create in pthreads
-- Looking for pthread_create in pthreads - not found
-- Looking for pthread_create in pthread
-- Looking for pthread_create in pthread - found
-- Found Threads: TRUE
-- Boost version: 1.58.0
-- Found the following Boost libraries:
--   system
--   filesystem
--   thread
--   date_time
--   chrono
--   regex
--   serialization
--   program_options
--   atomic
-- Found HIDAPI: /usr/lib/x86_64-linux-gnu/libhidapi-libusb.so
-- Configuring directory /root/openmonero/build/sql
-- Configuring file openmonero.sql
-- Configuring file openmonero_test.sql
-- Configuring done
-- Generating done
-- Build files have been written to: /root/openmonero/build
root@a2b653fe37f6:~/openmonero/build# make
Scanning dependencies of target myxrmcore
[  3%] Building CXX object src/xmregcore/src/CMakeFiles/myxrmcore.dir/MicroCore.cpp.o
[  6%] Building CXX object src/xmregcore/src/CMakeFiles/myxrmcore.dir/tools.cpp.o
[  9%] Building CXX object src/xmregcore/src/CMakeFiles/myxrmcore.dir/UniversalIdentifier.cpp.o
In file included from /usr/include/boost/assert.hpp:54:0,
                 from /usr/include/boost/date_time/time_facet.hpp:21,
                 from /usr/include/boost/date_time/posix_time/posix_time_io.hpp:16,
                 from /usr/include/boost/date_time/posix_time/posix_time.hpp:31,
                 from /root/monero/contrib/epee/include/net/network_throttle.hpp:65,
                 from /root/monero/contrib/epee/include/net/network_throttle-detail.hpp:40,
                 from /root/monero/contrib/epee/include/net/abstract_tcp_server2.h:59,
                 from /root/monero/contrib/epee/include/net/http_server_impl_base.h:36,
                 from /root/monero/src/wallet/message_transporter.h:35,
                 from /root/monero/src/wallet/message_store.h:46,
                 from /root/monero/src/wallet/wallet2.h:65,
                 from /root/openmonero/src/xmregcore/src/monero_headers.h:29,
                 from /root/openmonero/src/xmregcore/src/MicroCore.h:8,
                 from /root/openmonero/src/xmregcore/src/UniversalIdentifier.hpp:3,
                 from /root/openmonero/src/xmregcore/src/UniversalIdentifier.cpp:1:
/root/openmonero/src/xmregcore/src/Account.h: In member function 'const auto& xmreg::Account::keys()':
/root/openmonero/src/xmregcore/src/Account.h:93:32: error: cannot convert 'boost::optional<epee::mlocked<tools::scrubbed<crypto::ec_scalar> > >' to 'bool' in initialization
             assert(bool{viewkey});
                                ^
/root/openmonero/src/xmregcore/src/UniversalIdentifier.cpp: In member function 'virtual void xmreg::Output::identify(const cryptonote::transaction&, const crypto::public_key&, const std::vector<crypto::public_key>&)':
/root/openmonero/src/xmregcore/src/UniversalIdentifier.cpp:179:44: error: cannot convert 'std::unique_ptr<cryptonote::subaddress_index>' to 'bool' in initialization
             mine_output = bool {subaddr_idx};
                                            ^
/root/openmonero/src/xmregcore/src/UniversalIdentifier.cpp:201:48: error: cannot convert 'std::unique_ptr<cryptonote::subaddress_index>' to 'bool' in initialization
                 mine_output = bool {subaddr_idx};
                                                ^
src/xmregcore/src/CMakeFiles/myxrmcore.dir/build.make:88: recipe for target 'src/xmregcore/src/CMakeFiles/myxrmcore.dir/UniversalIdentifier.cpp.o' failed
make[2]: *** [src/xmregcore/src/CMakeFiles/myxrmcore.dir/UniversalIdentifier.cpp.o] Error 1
CMakeFiles/Makefile2:222: recipe for target 'src/xmregcore/src/CMakeFiles/myxrmcore.dir/all' failed
make[1]: *** [src/xmregcore/src/CMakeFiles/myxrmcore.dir/all] Error 2
Makefile:83: recipe for target 'all' failed
make: *** [all] Error 2
```

I've tried the `devel` branch of OpenMonero using `master` branch of Monero, and `master` of OpenMonero using `release-v0.14` of Monero. Both are failed with the errors above.

Any idea what I'm doing wrong?