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