On some autoconf versions (e.g. 2.71 on MacOS, installed with `brew 
install autoconf`), not quoting this argument results in a syntax error
in `configure`:

```
sh configure
# ...
checking for joinable pthread attribute... PTHREAD_CREATE_JOINABLE
checking whether more special flags are required for pthreads... no
checking for PTHREAD_PRIO_INHERIT... yes
configure: line 14927: syntax error near unexpected token `newline'
configure: line 14927: `    '''
```

Adding `[...]` quotes fixes this; see 
https://gerrit.openafs.org/#/c/9995/ for a similar problem in a
different OSS project.