meson: Make sure all install paths are absolute

libexecdir et al are all relative to prefix.
3 jobs from master in 2 minutes and 27 seconds
Status Job ID Name Coverage
  Test
passed #29015
ubuntu-16.04

00:01:56

passed #29016
ubuntu-17.10

00:02:12

passed #29017
ubuntu-18.04

00:02:26