qt-build-utils: allow QMAKE to be a script

[SanderVocke]

qt-build-utils expects QMAKE to point to the qmake binary. However, some Qt installations come with wrappers for qmake.

Example: qt built with vcpkg comes with a qmake.debug.bat in the same folder as qmake.exe, which wraps it to point to the debug build of Qt.

qt-build-utils currently cannot handle passing such a script as QMAKE, because it uses Command::new to execute it - and this only works with binaries, not with scripts.

I suppose to invoke cmd on the passed QMAKE instead, so that this use-case works. If you agree, I would be happy to provide a pull request.

An alternative solution would be to add another env var, QTCONF, which points to the qt conf file for qmake to use.

[LeonMatthesKDAB]

Hm, I didn't know it only works with binaries 🤔

Using cmd could be a reasonable approach (even the std docs use suggest this).
I'm unsure how this interacts with the actual arguments we provide to qmake.
But if you're confident you can work it out I'm happy to accept a PR.

[SanderVocke]

Created #1284. I do need to test some more configurations.

[LeonMatthesKDAB]

Alright, until you're confident that it works as intended, please convert the PR to a draft so we know when we need to review, thank you :)

[SanderVocke]

It is ready for review now, thanks!