configure allows specifying custom include flags to be used when building Qt with -I, custom defines with -D, custom link flags with -L, custom framework flags with -F.

These are currently used in the qmake Qt builds in the CI to specify OpenSSL include paths and libs. For CMake builds we handle that manually, by giving an OpenSSL_ROOT dir.

We should be able to handle those flags in CMake as well though (in cmake/QtProcessConfigureArgs.cmake)

This means introducing some CMake variables to contain those flags, writing those values either to QtBuildInternalsExtra.cmake (assuming they are only used for internal Qt builds, but I think that's not the case), or to Qt6Config.cmake / Platform target to make the public.

They also need to be written to the appropriate .pri / .prl files for qmake consumers.