Fix Qt4 removal from MSYS2 packages

[in3otd]

...trying a "fix" from here.

[in3otd]

Just noticed that the Appveyor docs say that "By default, saving cache is disabled in Pull Request builds.", so it seems we need to have this merged before having a valid cache.

[in3otd]

ok, it seems to work.
Someone please press the Merge button - or I'll do that later 😁.
Not sure this should go to master now, as there are other small things that should be merged in develop, so maybe we can merge develop into master later.

[guitorri]

Only releases branches should merge back into master.
I will give another try on finishing off release 20.

[in3otd]

thanks.
yes, but I wonder(ed) if also #953 and #951 (and #946, assuming I'll be able to finish it soon) should be included in 0.0.20.

I was also taking a look at how to do the release, cross-compiling for Windows in Linux - but I'll need quite some time to set up everything and I'm not sure I'll be able to make it work.

[guitorri]

Sure, tag them for 20. After they are in, we release it.