Allow custom dflags to be defined for dependencies

[omerfirmak]

For example;

{
	"authors": [
		"omer"
	],
	"copyright": "Copyright © 2020, omer",
	"description": "A simple vibe.d server application.",
	"license": "proprietary",
	"name": "dubtest",

	"dependencies": {
		"vibe-d": { "version" : "~>0.9.2", "dflags" : ["-preview=in"] }
	}
}

-preview=in will be applied to vibe-d and all of its dependencies. Any BuildSettings field can be used, parsing support is present. But, for now only, dflags is taken into account when compiling.

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[omerfirmak]

Would this need a changelog entry?

[Geod24]

Definitely. CC @s-ludwig , how do you feel about this ? The issue is that currently, we cannot propagate flags from the parent packages to the child packages. Things like -fPIC or -preview switches. I mentioned it in the DConf talk.

[Geod24]

And a test :)

[Geod24]

Using -version is a pretty bad example, because DUB handles it, and will give you a warning on this.
Use something like -fPIC instead, something that needs to be propagated and won't go away.

[omerfirmak]

I dont see any warning while building, at which point should I receive it? Both depen1 and depend2 need -version=depen_build_settings to successfully compile. If I remove or change it to something else, compilation fails as expected.

depen1 depends on depen2, so test also checks that the flags are applied transitively to all of the dependency chain.

[Geod24]

{
    "name": "mytest",
    "dflags": [ "-version=Hello" ]
}

Leads to:

$ dub build                                                                                                                                                          :(

## Warning for package mytest ##

The following compiler flags have been specified in the package description
file. They are handled by DUB and direct use in packages is discouraged.
Alternatively, you can set the DFLAGS environment variable to pass custom flags
to the compiler, or use one of the suggestions below:

-version=Hello: Use "versions" to specify version constants in a compiler independent way

Performing "debug" build using dmd for x86_64.
mytest ~master: building configuration "application"...
Linking...

[Geod24]

Also the test needs to make sure that the dflags is transitive otherwise it's not of much use to the user.

[Geod24]

Would solve #279

[andre2007]

Changing the dflags of a dependency should also trigger a new compilation of the dependency. Maybe a check for this could also be added to the test.

[Geod24]

LGTM, any opinion @PetarKirov @s-ludwig @wilzbach ?

[Geod24]

Suggested change

	`
	{
	"description": "A simple vibe.d server application.",
	"license": "proprietary",
	"name": "dubtest",
	"dependencies": {
	"vibe-d": { "version" : "~>0.9.2", "dflags" : ["-preview=in"] }
	}
	}
	`
	---
	{
	"description": "A simple vibe.d server application.",
	"license": "proprietary",
	"name": "dubtest",
	"dependencies": {
	"vibe-d": { "version" : "~>0.9.2", "dflags" : ["-preview=in"] }
	}
	}
	---

And there seems to be some tabs in there

[Geod24]

Fixed the changelog (and slightly improved it), merging since no opposition was voiced.

[Geod24]

@omerfirmak : This is missing the implementation for SDL.