build: Gather common path variables in root meson.build

This eliminates a lot of get_option and some join_paths.
3 jobs for master in 3 minutes and 7 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Test
passed #31592
ubuntu-16.04

00:02:55

passed #31593
ubuntu-17.10

00:03:06

passed #31594
ubuntu-18.04

00:02:43