Qt 5.15.9 mkspecs configuration files for building Qt 5.15 on a Raspberry Pi.

Common GL library configuration is in common/raspberrypi.conf
Model specific CFLAGS are set in linux-rpi2-g++, linux-rpi3-g++ and linux-rpi-g++
Configuration helpers for armv6, armv7 and armv8 configurations

Select a suitable configuration with -platform parameter to Qt configure. For detailed instructions on how to build Qt 5.15.10 for a Raspberry Pi see

https://www.tal.org/tutorials/building-qt-515-raspberry-pi

You can download Qt 5.15.10 sources from:

https://download.qt.io/official_releases/qt/5.15/5.15.10/single/