meson: Improve api documentation build file

the `doc_module` variable has been removed. It was created because
its used in the autotools build file but actually `nm_name` variable
can be used easily.

Different objects used in the documentation target have been grouped
together.

The content file `version.xml`, and different build files are now
added properly.

docs/api/meson.build

@@ -1,45 +1,52 @@
-doc_module = nm_name
-
-settings = 'settings-spec'
-output = settings + '.xml'
-
 if enable_introspection
-  settings_xml = custom_target(
+  settings = 'settings-spec'
+  output = settings + '.xml'
+
+  inputs = [
+    settings + '.xsl',
+    nm_settings_docs,
+  ]
+
+  content_files += custom_target(
     output,
-    input: nm_settings_docs,
+    input: inputs,
     output: output,
-    command: [xsltproc, '--output', '@OUTPUT@', join_paths(meson.current_source_dir(), settings + '.xsl'), '@INPUT@'],
-    # FIXME: there is no target depending on this, so it will not be built
+    command: [xsltproc, '--output', '@OUTPUT@', '@INPUT0@', '@INPUT1@'],
     build_by_default: true,
   )
 endif
 
-configure_file(
+content_files += configure_file(
   input: 'version.xml.in',
   output: '@BASENAME@',
   configuration: version_conf,
 )
 
 filecopier = find_program('cp')
-foreach intro : introspection_files
-  path = intro[1].split('/')[-1]
-  custom_target(path,
-                input: intro[0],
-                output: path,
-                command: [filecopier, intro[1], '@OUTPUT@'],
-                build_by_default: true,
+foreach intro: introspection_files
+  output = intro[1].split('/')[-1]
+
+  content_files += custom_target(
+    output,
+    input: intro[0],
+    output: output,
+    command: [filecopier, intro[1], '@OUTPUT@'],
+    build_by_default: true,
   )
 endforeach
 
-custom_target('common.ent',
-              input: common_ent_file,
-              output: 'common.ent',
-              command: [filecopier, '@INPUT@', '@OUTPUT@'],
-              build_by_default: true,
+output = 'common.ent'
+
+content_files += custom_target(
+  output,
+  input: common_ent_file,
+  output: output,
+  command: [filecopier, '@INPUT@', '@OUTPUT@'],
+  build_by_default: true,
 )
 
 gnome.gtkdoc(
-  doc_module,
+  nm_name,
   main_xml: 'network-manager-docs.xml',
   src_dir: meson.current_source_dir(),
   content_files: content_files,