games/freeorion: unbreak with boost 1.69
CMake Error at /usr/local/share/cmake/Modules/FindBoost.cmake:2049 (message):
Unable to find the requested Boost libraries.
Boost version: 1.69.0
Boost include path: /usr/local/include
Could not find the following Boost libraries:
boost_signals
Some (but not all) of the required Boost libraries were found. You may
need to install these additional Boost libraries. Alternatively, set
BOOST_LIBRARYDIR to the directory containing Boost libraries or BOOST_ROOT
to the location of Boost.Call Stack (most recent call first):
CMakeLists.txt:198 (find_package)
PR: 232525
Obtained from: upstream