Document supported version of FLTK

[chris-martin]

When I first tried to build with Nix, I got this failure:

==Compiling C bindings==
*** Compiling Fl_Simple_TerminalC.cpp...
In file included from Fl_Simple_TerminalC.cpp:1:0:
Fl_Simple_TerminalC.h:8:10: fatal error: FL/Fl_Simple_Terminal.H: No such file or directory
 #include "FL/Fl_Simple_Terminal.H"
          ^~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
make[1]: *** [Makefile:147: Fl_Simple_TerminalC.o] Error 1
make: *** [Makefile:16: src] Error 2
builder for '/nix/store/8aqr0dwp6cfkkqnbjdsm457vbbaqj4q4-fltkhs-0.6.0.0.drv' failed with exit code 2

If I upgrade the version of the nix fltk package to 1.4.x-r13121, then it seems to compile successfully.

Is it fair to conclude that this library requires fltk version >= 1.4, and can we add that to the readme?

[deech]

Yes it does require the development version of FLTK. I'll add it to the README since other people missed this as well so it's obviously not prominent enough.

[deech]

Thanks for the help with Nix packaging, learned a lot looking at your PR!