fails to compile extern ai_func not found

[apassiou]

emcc -O3 rftg.c -o rftg.bc
emcc: warning: generating an executable with an object extension (.bc). If you meant to build an object file please use -c, -r, or -shared` [-Wemcc]
wasm-ld: error: /tmp/emscripten_temp_sipg9qtk/rftg_0.o: undefined symbol: ai_func
wasm-ld: error: /tmp/emscripten_temp_sipg9qtk/rftg_0.o: undefined symbol: ai_func
wasm-ld: error: /tmp/emscripten_temp_sipg9qtk/rftg_0.o: undefined symbol: ai_func
wasm-ld: error: /tmp/emscripten_temp_sipg9qtk/rftg_0.o: undefined symbol: ai_func
wasm-ld: error: /tmp/emscripten_temp_sipg9qtk/rftg_0.o: undefined symbol: ai_func
wasm-ld: error: /tmp/emscripten_temp_sipg9qtk/rftg_0.o: undefined symbol: ai_func
wasm-ld: error: /tmp/emscripten_temp_sipg9qtk/rftg_0.o: undefined symbol: ai_func
wasm-ld: error: /tmp/emscripten_temp_sipg9qtk/rftg_0.o: undefined symbol: ai_func
wasm-ld: error: /tmp/emscripten_temp_sipg9qtk/rftg_0.o: undefined symbol: ai_func
wasm-ld: error: /tmp/emscripten_temp_sipg9qtk/rftg_0.o: undefined symbol: ai_func
wasm-ld: error: /tmp/emscripten_temp_sipg9qtk/rftg_0.o: undefined symbol: num_campaign
wasm-ld: error: /tmp/emscripten_temp_sipg9qtk/rftg_0.o: undefined symbol: camp_library

$ emcc --version
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 2.0.26 (5af6a110ee58ca4a6885fbcad830c92207fefea2)
Copyright (C) 2014 the Emscripten authors (see AUTHORS.txt)
This is free and open source software under the MIT license.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ convert --version
Version: ImageMagick 6.9.10-23 Q16 x86_64 20190101 https://imagemagick.org
Copyright: © 1999-2019 ImageMagick Studio LLC
License: https://imagemagick.org/script/license.php
Features: Cipher DPC Modules OpenMP
Delegates (built-in): bzlib djvu fftw fontconfig freetype jbig jng jpeg lcms lqr ltdl lzma openexr pangocairo png tiff webp wmf x xml zlib

$ ls -lh
total 16M
-rwxrwxrwx 1 user user 63 May 31 2016 AUTHORS
-rwxrwxrwx 1 user user 18K May 31 2016 COPYING
-rwxrwxrwx 1 user user 1.9K May 31 2016 Makefile
-rwxrwxrwx 1 user user 3.5K May 31 2016 README
-rwxrwxrwx 1 user user 1.4K May 31 2016 README.md
-rwxrwxrwx 1 user user 189K May 31 2016 ai.c
-rwxrwxrwx 1 user user 11K May 31 2016 ai_client.c
-rwxrwxrwx 1 user user 11K Nov 25 2016 campaign.txt
-rwxrwxrwx 1 user user 34K Nov 25 2016 cards.txt
-rwxrwxrwx 1 user user 67K May 31 2016 client.c
-rwxrwxrwx 1 user user 4.1K May 31 2016 client.h
-rwxrwxrwx 1 user user 3.4K May 31 2016 comm.c
-rwxrwxrwx 1 user user 2.1K May 31 2016 comm.h
-rwxrwxrwx 1 user user 537 May 31 2016 do_train
-rwxrwxrwx 1 user user 1.7K May 31 2016 dumpnet.c
-rwxrwxrwx 1 user user 289K May 31 2016 engine.c
drwxrwxrwx 1 user user 4.0K May 31 2016 fonts
-rwxrwxrwx 1 user user 355K May 31 2016 gui.c
-rwxrwxrwx 1 user user 1.3K May 31 2016 icon.svg
-rwxrwxrwx 1 user user 14M Nov 25 2016 images.data
-rwxrwxrwx 1 user user 24K May 31 2016 init.c
-rwxrwxrwx 1 user user 5.0K May 31 2016 learner.c
-rwxrwxrwx 1 user user 19K May 31 2016 loadsave.c
-rwxrwxrwx 1 user user 19K May 31 2016 net.c
-rwxrwxrwx 1 user user 2.7K May 31 2016 net.h
-rwxrwxrwx 1 user user 1.4K May 31 2016 netconv.c
drwxrwxrwx 1 user user 4.0K May 31 2016 network
-rwxrwxrwx 1 user user 1.6K May 31 2016 rftg.appcache.in
-rwxrwxrwx 1 user user 62K May 31 2016 rftg.c
-rwxrwxrwx 1 user user 29K May 31 2016 rftg.h
-rwxrwxrwx 1 user user 85K May 31 2016 rftg.html
-rwxrwxrwx 1 user user 473 May 31 2016 rftg.manifest.json
-rwxrwxrwx 1 user user 100K May 31 2016 server.c