r273910 build failure if <sys/_umtx.h> is out-of-date

[Beeblebrox]

Hello

> Beeblebrox should confirm whether or not he’s using gcc in the build
> or clang. Thanks!

Using clang
I have some tighter settings in src.conf, but nothing clang/gcc related.

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FreeBSD_amd64_11-Current_RadeonKMS