build: use join_paths() for doxygen paths

12 jobs for master in 7 minutes and 54 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #239039
amd64 debian
build-debian

00:00:34

passed #239043
debian aarch64
build-debian-aarch64

00:01:25

passed #239044
debian aarch64
build-debian-aarch64-clang-5

00:01:02

passed #239040
amd64 debian
build-debian-static

00:00:32

passed #239046
debian aarch64
build-debian-werror

00:00:33

passed #239045
macos
build-macos

00:00:25

passed #239041
win32
build-win32

00:00:29

passed #239042
win64
build-win64

00:00:34

 
  Test
passed #239047
amd64 debian
test-debian

00:00:48

passed #239048
amd64 debian
test-debian-asan

00:02:16

passed #239049
amd64 debian
test-debian-msan

00:01:09

passed #239050
amd64 debian
test-debian-ubsan

00:01:28