"cannot represent a NaN literal" while building shared lib with QtGui

I am experimenting with Qt for WebAssembly, which I installed as per these instructions (Qt 5.12.0, emscripten 1.38.16). The attached script produces an example with a minimal .cpp file and a related .pro file. Running it leads to warnings similar to the following:

emcc: warning: cannot represent a NaN literal '0xc2e65e0' with custom bit pattern in NaN-canonicalizing JS engines (e.g. Firefox and Safari) without erasing bits!

and errors like the following (as described here):

error: undefined symbol: png_create_info_struct

Some observations:
1. The reason why I am using "CONFIG *= shared" is that I am not building an app, but instead using embind;

2. The errors and warnings are related to "QT = core widgets gui": "QT = core" alone works with no errors;

3. By performing the linking step manually I could get rid of the errors. However, the resulting .wasm is huge (8 MB+) and does not seem to work correctly.