configure.ac: fix a syntax error

Fixes this error:

  checking whether more special flags are required for pthreads... no
  checking for PTHREAD_PRIO_INHERIT... yes
  ./configure: line 30294: ,as_fn_error: command not found
  checking for a Python interpreter with version >= 3... python
  checking for python... /usr/bin/python
57 jobs for lr/ac-threads in 27 minutes and 8 seconds (queued for 3 seconds)
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