Allow to add additional DFLAGS when building Phobos

[wilzbach]

dlang/dmd#8089 for Phobos, s.t. downstream packers can set their custom flags and don't need to patch posix.mak like e.g. #5966

It's also useful for quickly applying a flag for local testing:

make -f posix.mak std/algorithm/iteration.test DFLAGS="-debug=MyDebug"

For reference, environment variables aren't read, so global DFLAGS won't start to influence this Makefile, e.g.

DFLAGS="-debug=MyDebug" make -f posix.mak std/algorithm/iteration.test

[dlang-bot]

Thanks for your pull request, @wilzbach!

Bugzilla references

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If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + phobos#6392"

[wilzbach]

FYI the analog DMD PR has been merged a few days ago now.

[edi33416]

Nice!

[edi33416]

@wilzbach do you think this change would require a changelog or an announcement on the forum? I'm just wondering if there were any folks relying on the previous behaviour: If a variable has been set with a command argument [..], then ordinary assignments in the makefile are ignored.

The same question for the dmd pr