meson: replace deprecated meson.source_root

Instead, meson.current_source_root or meson.project_source_root should
be used:
https://mesonbuild.com/Reference-manual_builtin_meson.html#mesonsource_root

Also, the documentation referenced above suggest to use `files()` as a
better alternative to refer to files, so do that at the same time.

This gets rid of the deprecation warning:
  NOTICE: Future-deprecated features used:
   * 0.56.0: {'meson.source_root'}

man/meson.build

@@ -62,7 +62,7 @@ foreach man: mans
 endforeach
 
 if enable_introspection
-  merge_cmd = join_paths(meson.source_root(), 'tools', 'generate-docs-nm-settings-docs-merge.py')
+  merge_cmd = files(source_root / 'tools' / 'generate-docs-nm-settings-docs-merge.py')
 
   name = 'dbus'
   nm_settings_docs_xml_dbus = custom_target(

meson.build

@@ -77,7 +77,7 @@ pkg = import('pkgconfig')
 source_root = meson.current_source_dir()
 build_root = meson.current_build_dir()
 
-po_dir = join_paths(meson.source_root(), 'po')
+po_dir = source_root / 'po'
 
 top_inc = include_directories('.')
 

src/libnm-client-impl/meson.build

@@ -177,8 +177,8 @@ if enable_introspection
   endif
   ld_library_path = meson.current_build_dir() + ld_library_path
 
-  gen_infos_cmd = join_paths(meson.source_root(), 'tools', 'generate-docs-nm-property-infos.py')
-  gen_gir_cmd = join_paths(meson.source_root(), 'tools', 'generate-docs-nm-settings-docs-gir.py')
+  gen_infos_cmd = files(source_root / 'tools' / 'generate-docs-nm-property-infos.py')
+  gen_gir_cmd = files(source_root / 'tools' / 'generate-docs-nm-settings-docs-gir.py')
 
   names = [ 'dbus', 'nmcli', 'keyfile' ]
   if enable_ifcfg_rh

src/libnm-client-impl/tests/meson.build

@@ -50,9 +50,9 @@ if enable_introspection
     'check-local-libnm-gir',
     python,
     args: [
-      join_paths(meson.source_root(), 'src', 'libnm-client-impl', 'tests', 'test-gir.py'),
+      join_paths(source_root, 'src', 'libnm-client-impl', 'tests', 'test-gir.py'),
       '--gir', libnm_gir[0],
-      '--ver', join_paths(meson.source_root(), 'src', 'libnm-client-impl', 'libnm.ver'),
+      '--ver', join_paths(source_root, 'src', 'libnm-client-impl', 'libnm.ver'),
     ],
   )
 endif

src/libnmc-setting/meson.build

@@ -3,7 +3,7 @@
 if enable_docs
   assert(enable_introspection, '-Ddocs=true requires -Dintrospection=true')
 
-  merge_cmd = join_paths(meson.source_root(), 'tools', 'generate-docs-nm-settings-docs-merge.py')
+  merge_cmd = files(source_root / 'tools' / 'generate-docs-nm-settings-docs-merge.py')
   settings_docs_input_xml = custom_target(
     'settings-docs-input.xml',
     input: [merge_cmd, nm_settings_docs_xml_gir['nmcli'], nm_property_infos_xml['nmcli']],
@@ -17,7 +17,7 @@ if enable_docs
     ],
   )
 
-  gen_cmd = join_paths(meson.source_root(), 'tools', 'generate-docs-settings-docs.py')
+  gen_cmd = files(source_root / 'tools' / 'generate-docs-settings-docs.py')
   settings_docs_source = custom_target(
     'settings-docs.h',
     input: [gen_cmd, settings_docs_input_xml],