ostinato: make build more standard

This is just a proposal, it won't fix any bugs.

I don't like the current complex make command in build(), I would like a more standard one. To achieve that, I patched the QMAKE_CXXFLAGS in options.pri instead of using CXXFLAGS in the make command.

Futhermore I prefer the Debian modeltest patch https://salsa.debian.org/debian/ostinato/blob/master/debian/patches/extra-modeltest-workaround.diff, so I integrated that patch.

Here my patch: ostinato_alpine.diff
I tested that only with alpine v3.12.1 on x86_64.

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