qt-build-system: Add QT_QPA_PLATFORM=offscreen?

[Hartmut Goebel <h.goebel@crazy-compilers.com>]

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Hi Danny,

(forwarding to guix-devel to eventually get more opinions on this)

Am 21.01.20 um 23:43 schrieb Danny Milosavljevic:
>> +         (add-before 'check 'check-setup
>> +           (lambda _
>> +             (setenv "QT_QPA_PLATFORM" "offscreen")
>> +             #t)))))
> Hmm, would it make sense to do that automatically in qt-build-system ?

I would be fine with this, since I find myself adding this snippet quite
often when building Qt/KDE applications. But there *might* be cases,
where tests fail due to this setting. (While I'm not aware of one, I
could imaging some test checks whether it is actually painting.)

Thus I propose implementing this in some way to easily disable it if
required: Either

- by adding this a new phase (as in the snippet shown above), so it can
easily be disabled

- by adding another keyword-argument (proposal: `#:qpa-platform`,
default: "offscreen", if +f, will not be set.

WDYT?

-- 
Regards
Hartmut Goebel

| Hartmut Goebel          | h.goebel@crazy-compilers.com               |
| www.crazy-compilers.com | compilers which you thought are impossible |



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