meson: add prefix to serverconfigdir

fixes #853
3 jobs for patch-1 in 8 minutes and 7 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #1368028
debian-testing

00:01:34

 
  Build And Test
passed #1368029
autotools-build-and-test

00:06:32

passed #1368030
meson-build-and-test

00:02:31