Hi,
thank you for your post.
It seems that you may have built (and maybe installed) fuzzylite using floats instead of doubles, that is, with compiler option -DFL_USE_FLOAT.
You have to make sure that the library you are linking against is built without -DFL_USE_FLOAT. Moreover, please check if you have installed fuzzylite in your system (e.g. /usr/local/[include|lib]/), that is, if at some point you did sudo make install fuzzylite.
If you are going to be using floats instead of doubles, then you have to pass the compiler option -DFL_USE_FLOAT to your project too because you #include <fl/Headers.h>.
Let me know if this helps.
Cheers.