Do you want a kiwix-specific env var to control the build behavior? The QT_SELECT env var is not the only way to select which Qt installation to use, so someone might be intentionally using a qt5 toolchain yet the build would fail because the QT_SELECT env var was not set. Similarly, if someone already set QT_SELECT=qt5 as part of their default shell environment, the build wouldn't fail even though maybe it should have.

A kiwix-specific var like KIWIX_QT=qt5 would prevent this.

I have no strong opinion on this point... I mostly focus on simplicity. Just to be sure I understand your scenarios:

someone might be intentionally using a qt5 toolchain yet the build would fail because the QT_SELECT env var was not set.

OK, but then he clearly ignores the README right? If he ignores the README he won't find out how to set KIWIX_QT=qt5 either.

Similarly, if someone already set QT_SELECT=qt5 as part of their default shell environment, the build wouldn't fail even though maybe it should have.

You mean, he would have set QT_SELECT=qt5 but actually compiling with Qt6 for example? OK... but that sounds pretty weird!

OK, but then he clearly ignores the README

I typically assume folks ignore the README, at least to some degree. They will usually follow any initial instructions but I would not expect beginners to dig into the details of the README. Even if they read it, they don't often know the significance of what they read, or what context it applies. Just saying there are limits to what beginners will pick up on.

With KIWIX_QT=qt5, my understanding was that you wanted a default Qt6 experience, where everyone compiles with qt6 by default, and only drops back to qt5 for certain cases like compiling on older distros. If that's what you want, using a kiwix-specific env var would accomplish that.

Regarding my original comment - someone might be intentionally using a qt5 toolchain - what I mean is that they could use a different avenue of configuring qtchooser, like via a config file. They could have intentionally setup their environment to build with Qt5, but the build would fail because they configured qtchooser via config instead of QT_SELECT. This is not a huge deal, but could trip up someone who doesn't understand the intent of the environment variables. This is why I think KIWIX_QT is more clear all around. If we use QT_SELECT to determine whether a Qt5 build is allowed, we're conflating the configuration of qtchooser ("which version of Qt does the system provide") with the requirements of kiwix ("which version of Qt does the project require").

he would have set QT_SELECT=qt5 but actually compiling with Qt6

No, I mean that a Qt developer with a preexisting default of QT_SELECT=qt5 in their environment could walk into the project and accidentally compile with Qt5. If I understand what you want - compile with Qt6 by default except some limited circumstances - this scenario seems to break your requirement. Also not a huge deal, but I bring it up because it seems contrary to the desired experience.