Ok now in 2.0RC1 your way works, but the size of the binary is still huge compared to the one in the base system.
g++ -Os -fno-exceptions -fno-rtti -lfltk -o add2file add2file.cxx
strip add2file
results an add2fle of 386824 KB, where the one in the base system is just 4656 KB
The thing is that I just grabbed this app from the src dir from ibiblio, actually from the nluug.nl miror, since I'm in europe, and just compiled and stripped, without modifying either the .ccx or the .h