[build.d] Print stdout for dmd

[wilzbach]

CC @andralex

Downside is that it prints the TLS warning twice now. Maybe only in verbose mode?

>  ./build.d
(TX) DMD_CONF
(DC) BACKEND
(DC) LEXER
[lexer] /usr/include/dlang/dmd/core/exception.d(631): `_store` is thread local
(DC) DMD
[dmd] /usr/include/dlang/dmd/core/exception.d(631): `_store` is thread local
Success

[dlang-bot]

Thanks for your pull request, @wilzbach!

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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#11352"

[andralex]

Just making sure - that prints stderr not stdout, right?

[andralex]

Suggested change

	output.splitter(newline).map!(e => "[" ~ ruleName ~ "] " ~ e).joiner(newline).writeln;
	output.splitter(newline).each!(ln => writeln('[', ruleName, "] ", ln));

is simpler I guess

[wilzbach]

This risks interleaving with other log messages from other processes (build.d might executes multiple dependencies at the same time).

[wilzbach]

Just making sure - that prints stderr not stdout, right?

It prints both - https://dlang.org/phobos/std_process.html#.execute

[MoonlightSentinel]

Not sure I like this special case, I would rather make the following changes:

1. Always print the entire output in verbose mode.
2. Only print stderr in default mode
   This could be achieved by either passing Config.stderrPassThrough to execute(allthough this would prevent a prefix) or using pipeProcess directly.

[wilzbach]

Closing in favor of #11641