patch-docs_reference_libnice_meson.build

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

--- docs/reference/libnice/meson.build.orig	2020-11-25 16:39:27 UTC
+++ docs/reference/libnice/meson.build
@@ -26,7 +26,7 @@ ignore_headers = [
 ]
 
 if dependency('gtk-doc', version: '<1.30', required: false).found()
-  prog_python = import('python').find_installation('python3')
+  prog_python = import('python').find_installation()
   fake_makefile = custom_target ('libnice-docs-test-Makefile',
                                  output: 'Makefile',
                                  command: [
@@ -67,6 +67,8 @@ gnome.gtkdoc('libnice',
 # If we ever need to regenerate this diagram.
 # Since it’s not expected to change much, let’s not depend on GraphViz to
 # build the docs (that's also why we don't use find_program('dot') here)
+dot = find_program('dot', required: false)
+if dot.found()
 run_target('update-states.png',
   command: ['dot',
             '-Tpng',
@@ -74,3 +76,4 @@ run_target('update-states.png',
             '-Gsize=9.6,2.9!',
             '-Gdpi=200',
             files('states.gv')])
+endif