Keep rsync optional with meson >= 0.58 after
https://github.com/mesonbuild/meson/commit/80c89a650b6f

--- doc/meson.build.orig	2020-10-16 10:59:17 UTC
+++ doc/meson.build
@@ -18,6 +18,8 @@ if get_option('html_manual')
     install_dir: join_paths(get_option('datadir'), 'doc', meson.project_name()),
   )
 
+  rsync = find_program('rsync', required: false)
+  if rsync.found()
   custom_target(
     'upload',
     input: sphinx_output,
@@ -29,6 +31,7 @@ if get_option('html_manual')
       '--chmod=a+rX',
     ],
   )
+  endif
 endif
 
 if get_option('manual')