In your build, the linker is looking for ??1Fl_Widget@@UEAA@XZ which can be found in the 64-bit version, not the 32-bit version. Another confirmation that you are compiling your code as 64-bit but linking against a 32-bit version of fltk.lib.
I had difficulties making both the 32-bit and 64-bit versions of fltk because I was creating new configurations from within Visual Studio. I found that I had to check the Project Properties to confirm the settings were correct. For instance, I had to check both "Advanced \ Preferred Build Tool Architecture" AND "Librarian \ Additional Options" to insure consistent settings.
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