FindFLTK: Find both debug and release libraries

in vcpkg, debug libraries are built inside a debug/ folder and appended to the path looked by cmake. Since this script looks for fltk.lib before looking for fltkd.lib, it means that it will find fltk (non debug) also when requested the debug library. Reverting the search order fixes the problem: it will find fltkd.lib when necessary and it will find fltk.lib when the debug folder is not appended to the search path. I don't know of any drawback for other systems The whole script seems a little bit "old", but this minimal fix should be enough to solve the problem without interfering too much.

https://github.com/Microsoft/vcpkg/issues/1320

Topic-rename: FindFLTK-per-config