The project is archived. Nowdays orignal game works fine in js-dos https://dos.zone/transport-tycoon-deluxe-1995/ and OpenTTD itself have a support for emscripten.

1. Build host version:

mkdir build-host
cd build-host
cmake ../OpenTTD
make openttd

2. Build emscripten version:

mkdir emscripten
cd emscripten
emcmake cmake ../OpenTTD
make