cmake: Create all output directories for Doxygen

CI builds intermittently fail with

    error: Could not create output directory /.../doc/api/xml

or

    error: Could not create output directory /.../doc/api/man

Fixes: dbus/dbus#266
Signed-off-by: Simon McVittie <smcv@collabora.com>
16 jobs for cmake-mkdir in 12 minutes and 52 seconds (queued for 1 second)