Fix issue 20825: paths in error messages from build.d should be click…

[MoonlightSentinel]

…able.

Just to improve the current development process.

Before:

[...]
-----------------------------------------------------------
ERROR: dmd/expression.d(763): Error: undefined identifier `xxx`

After:

[...]
-----------------------------------------------------------
ERROR: src/dmd/expression.d(763): Error: undefined identifier `xxx`

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20825	normal	the filename of the error messages generated by dmd build.d script miss the "src/" part of the path

Testing this PR locally

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

dub run digger -- build "master + dmd#11157"

[Geod24]

Note that clickable for one is not clickable for the other... It always baffled me that the makefiles were in src, so I always assumed it was Walter's working directory.

[MoonlightSentinel]

Always thought that it was just a common pattern to split responsibilities in nested Makefiles.

But I can always make this optional if it causes problems for other people.

[Geod24]

No the one in src predates posix.mak by a long shot. I've seen quite a lot of old projects doing this.

In any case I'm very happy with the change, let's see if someone complains.