When compiling tests with bazel with clang on linux, I get a linker error undefined reference to nextafter.

Does the bug persist in the most recent commit?

Yes.

What operating system and version are you using?

Linux. The specifics don't matter because we're using a custom sysroot, described below.

What compiler and version are you using?

clag version 13.0.0 with --sysroot pointing to a sysroot built using crosstool-ng based on centos7.

What build system are you using?

bazel version 5.1.1.

Additional context

This is basically the same bug as #3601, but the fix for that one made the incorrect assumption that this was only a problem on BSD. The correct solution is to change

    "//conditions:default": ["-lm", "-pthread"], 

It won't do any harm when using compilers like gcc which add -lm implicitly, but it will prevent breakage for compilers which do not do this.