CMakeLists.txt will not change the flag. You need to pass
-DFL_USE_FLOAT=ON to CMake on command line.
cmake .. -G"Unix Makefiles" -DCMAKE_BUILD_TYPE=Release -DFL_BACKTRACE=ON -DFL_USE_FLOAT=ON -DFL_CPP11=ON
Or if you are using
build.sh|bat, please modify such lines accordingly.
Let me know if you make it work.
For more information, please visit: http://fuzzylite.com/cpp/#Advanced_building_options