There is an updated pull request by HadetTheUndying against master on the void-packages repository

https://github.com/HadetTheUndying/void-packages mesa-update
https://github.com/void-linux/void-packages/pull/27956

[wip] mesa: update to 21.0.0.
Reason for changing to the `-Dosmesa` argument:

https://gitlab.freedesktop.org/mesa/mesa/-/commit/ee802372180a2b4460cc7abb53438e45c6b6f1e4

Obviously i'll be fixing the distfiles and wrksrc changes when a stable release is out.

A patch file from https://github.com/void-linux/void-packages/pull/27956.patch is attached