meson: build reference manual with gtk_doc

docs/libpsl/meson.build

@@ -1 +1,20 @@
-install_man('libpsl.3')
+gnome = import('gnome')
+
+if get_option('docs')
+  configure_file(
+    input : 'version.xml.in',
+    output : 'version.xml',
+    configuration : cdata)
+
+  gnome.gtkdoc('libpsl',
+    main_sgml: 'libpsl-docs.sgml',
+    src_dir: [
+      'src',
+      'include',
+    ],
+    dependencies: libpsl,
+    scan_args: [
+      '--ignore-decorators=G_GNUC_PSL_UNUSED',
+    ],
+    install: true)
+endif

meson_options.txt

@@ -14,3 +14,6 @@ option('psl_file', type : 'string', value : '',
 
 option('psl_testfile', type : 'string', value : '',
   description : 'path to PSL test file')
+
+option('docs', type: 'boolean', value: false,
+  description: 'Build the API reference (requires gtk-doc)')