DUB has a "--build=release|unittest|profile|etc", but is there
currently any way to configure them? If not, there should be.

For example, on my projects, I typically wrap my unittest blocks in a
project-specific version identifier like version(MyProj_Unittest). Or
on other projects, there can be bugs that prevent them from working
right with certain flags like -inline. Or a project's docs may be
specifically designed/intended to be built a certain way or with a
certain particular other tool. So there needs to be a way to add/remove
options from those builds if there isn't already a way.