There's a closed pull request on the void-packages repository

[wip] mesa: update to 21.0.0.
https://github.com/void-linux/void-packages/pull/27956

Description:
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.