Back in 2010 there was a native Direct3D 10/11 implementation for Gallium3D that was implemented as a state tracker. This state tracker implemented the modern Direct3D APIs directly rather than relying upon any Direct3D to OpenGL conversion layer or other interfaces, but it was basically never used. There's no Linux user-space programs relying upon Direct3D and there's also legal uncertainty concerning this implementation.
For a while it looked like Wine might use the Direct3D state tracker, but that didn't end up happening with the state tracker and Mesa/Gallium3D not being supported everywhere.
Since then, there hasn't been much activity or real-world use surrounding this "d3d1x" state tracker. The state tracker has fallen into disrepair.
With a commit made on Thursday evening, Matt Turner has disabled this state tracker from being built unless explicitly specified. "It's broken and unmaintained, and I'm tired of seeing bug reports about it."
Should you care about this Direct3D state tracker for Gallium3D (in hopes of restoring work on it), pass --enable-d3d1x when building Mesa.