Configure options for deployment

[wysota]

release: I don't need to debug inside Qt

Right.

zlib and image formats: I guess Windows doesn't have these, at least with the API that Qt wants

Qt zlib is used by default on Windows, AFAIK.

CPU extensions: MMX is safe, but the libraries need to run on machines without an SSE-featuring CPU

As I understand it, it is support, not requirement so if you have support for SSE instructions they will simply not be used on machines not supporting them.

Are these assumptions safe and adequate to get portable libraries?

You can go with the defaults, they are fine, no needs for any switches.

I'm not really sure if I need to enable Zlib and especially MNG format, since I'm not sure where Qt uses them. Any advice on this?

Zlib is used at least with qCompress() and qUncompress(). It is also used by libpng, so if you want to have support for PNG (and you probably do) then you need zlib anyway.