Make sure that config.h is the first file included

It contains preprocessor macros that affect what functions are
enabled in the standard C headers (e.g. `strnlen`).
12 jobs for strnlen-configure in 35 minutes and 14 seconds (queued for 4 minutes and 8 seconds)
Status Job ID Name Coverage
  Level0
passed #31667
level0:lal

00:06:11

 
  Level1
passed #31668
level1:lalframe

00:02:26

passed #31669
level1:lalmetaio

00:01:58

passed #31670
level1:lalsimulation

00:06:16

passed #31671
level1:lalxml

00:01:55

 
  Level2
passed #31672
level2:lalburst

00:02:23

passed #31673
level2:lalinspiral

00:05:10

passed #31674
level2:lalpulsar

00:03:36

passed #31675
level2:lalstochastic

00:01:52

 
  Level3
passed #31676
level3:laldetchar

00:02:19

passed #31677
level3:lalinference

00:06:55

 
  Level4
passed #31678
level4:lalapps

00:06:41