There might be a couple of issues here:
1. An error coming from libfltk (maybe) not being (re)compiled against the updated libstdc++/libgcc_s in the base.
2. A mismatch between the fltk libs in the base and the dev files in base-dev, though I don't see anything obvious
If you recompile on tc-3 without "enable-threads" do you get the same error?
I get the same error compiling with TC base or with fltk libs replaced with threads-enabled. Did not recompile fltk with threads disabled, as basically fltk libs look OK in the base except threads and possibly xft, but it will come later.
All together threading issue is solved. Application itself need threeding. problem must be somwhere around libstdc++-libgcc and not fltk
If it helps, I can send you build script, fldigi source and recompiled fltk for debugging.