allow to configure DFLAGS and WARNINGS

[MartinNowak]

• e.g. to test dlang/tools without erroring on deprecations (ci.dlang.org)
• use override DFLAGS += for non-configurable target specific flags

[dlang-bot]

Thanks for your pull request, @MartinNowak!

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[wilzbach]

Makes sense, but we probably want to use -de on the auto-tester as it leaves a bad imagine if our core tools have deprecation messages and if they aren't fixed by the one who introduces the deprecation, no one will care.

[MartinNowak]

Makes sense, but we probably want to use -de on the auto-tester as it leaves a bad imagine if our core tools have deprecation messages and if they aren't fixed by the one who introduces the deprecation, no one will care.

Seems fair to push the deprecation cleanup effort for any dlang code onto the PR that introduces them.