[enh] Generic way to hijack dependencies' dflags

[Geod24]

There's currently no way to change a dependency dflags, other than changing the said dependency dub.json.
That is problematic for specific use case:

• If one wants to use shared libraries, the code has to be PIC. I'm currently writing a PAM module which use vibe.d, and I had to fork it, branch it, and add a libevent-pic configuration. So that currently forces library writer to provide at least 3 versions of said library: static, static PIC, dynamic.
• Aside from PIC, one may want to disable specific features (like "-mno-sse" on gdc).

It could be useful if we had a way to edit [d|l]flags (add / remove / do it from scratch) in the "subConfiguration".

[s-ludwig]

The PIC issue needs to be managed by DUB automatically, that's a bug in the way shared library builds are performed right now (they are basically completely untested on my part).

For adding flags like -mno-sse, using the "buildTypes" field should be possible right now, but I think there is nothing to remove certain flags (except for the ones controllable using "buildRequirements"). But that's just the status quo, it would indeed be good to have more control here.

[Geod24]

Solved by #2040