adwaita-qt: enable Qt 6 support

[ghost]

Testing the changes

• I tested the changes in this PR: briefly

Local build testing

• I built this PR locally for my native architecture, x86-64-glibc

[ghost]

Cross compile is failing because QT_HOST_PATH isn't set (required when cross compiling with Qt 6 as it turns out).
Not sure how to make this work, the best information I found about this variable are two messages sent to a Debian mailing list by someone who also didn't seem to figure this out.

[classabbyamp]

hostmakedepends+=qt6-tools

[classabbyamp]

also this seems to remove the qt5 version. it's probably going to have to be built twice, once with each version of qt

[ghost]

Yeah you're right I didn't realize that (I was using a local Qt 6 build alongside the current package so I had both versions installed).
Maybe this should be split into two packages?

[classabbyamp]

i'm not sure this needs separate packages, maybe something like what qwt does could be done

[sgn]

d2ade21