You can add it in the depending package (i.e. the final application package that gets built). Versions and debug versions are always propagated to the dependencies to ensure a consistent build.

This is sad that in order to build a library written by developers who prefer Dub, I have to use Dub, too. :)

You could also build with custom flags using: 

DFLAGS=-version=VibeCustomMain dub build

This will override the flags of the usual build mode (e.g. -g -debug).

Didn't know about this, thank you!

There is a targetPath setting for specifying where the final binaries are put. Or do you mean that you prefer to put the build results to a whole different location (e.g. ~/my_build_results/libvibe-d.a)?

I don't think that a project file (dub.json) is a good place for writing such machine-specific things. Since dub.json is usually included into VCS repository, it should not have hardcoded paths inside. IMHO, dub command should have arguments for defining such paths on individual developer's machine instead.

Of course, environment variables are a nice way to do it, too. DFLAGS seems to solve the problem, but is there a similar variable for defining output file?