configure.ac: Fix warning

6 jobs for master in 309 minutes and 35 seconds (queued for 134 minutes and 41 seconds)
Status Job ID Name Coverage
  Test
passed #2201
amd64
job_clang_amd64

02:57:45

passed #2205
amd64
job_clang_amd64_nosh

02:32:40

passed #2203
amd64
job_clang_py2_amd64

02:40:49

passed #2200
amd64
job_gcc_amd64

02:11:49

passed #2204
amd64
job_gcc_amd64_nosh

02:02:21

passed #2202
amd64
job_gcc_py2_amd64

02:10:44