Meson: Fix doc build when opus is a subproject

meson.source_root() and meson.build_root() have been deprecated in
latest Meson release because they are a trap. They point to the root of
parent project instead of root of current subproject. Meson 0.56.0 added
meson.project_source/build_root() but it is just as easy to use
meson.current_source/build_dir() in the root meson.build file and avoids
bumping required meson version.

Signed-off-by: Ralph Giles <giles@thaumas.net>