Since D4474 the unit tests rely on BOOST_TEST_NO_MAIN to override the
default Boost unit tests framework main function.
This doesn't work by default when Boost is built as a static library,
which is the case when the software builds against the depends
libraries.
This diff makes the Boost static build compliant for using with a custom
main, as per the documentation:
https://www.boost.org/doc/libs/1_64_0/libs/test/doc/html/boost_test/adv_scenarios/static_lib_customizations/entry_point.html.

For all platforms:

PLATFORM=Linux64 # Replace with the target platform
mkdir -p "build${PLATFORM}"
cd "build${PLATFORM}"
cmake -GNinja .. \
  -DCMAKE_TOOLCHAIN_FILE="../cmake/platforms/${PLATFORM}.cmake"
ninja test_bitcoin

Run the Gitian builds twice and ensure the builds are still
deterministic.