gdbus-codegen: Add --output-directory flag

This is useful with Meson where files are generated in subdirs

https://bugzilla.gnome.org/show_bug.cgi?id=778801

docs/reference/gio/gdbus-codegen.xml

@@ -33,6 +33,7 @@
     <arg><option>--c-namespace</option> <replaceable>YourProject</replaceable></arg>
     <arg><option>--c-generate-object-manager</option></arg>
     <arg><option>--c-generate-autocleanup</option> none|objects|all</arg>
+    <arg><option>--output-directory</option> <replaceable>OUTDIR</replaceable></arg>
     <arg><option>--generate-docbook</option> <replaceable>OUTFILES</replaceable></arg>
     <arg><option>--xml-files</option> <replaceable>FILE</replaceable></arg>
     <group choice="plain" rep="repeat">
@@ -187,7 +188,12 @@
         <para>
           Generate C code for all D-Bus interfaces and put it in
           <filename>OUTFILES.c</filename> and
-          <filename>OUTFILES.h</filename>.
+          <filename>OUTFILES.h</filename> including any sub-directories. If you want the files to
+          be output in a different location use <option>--output-directory</option> as <filename>OUTFILES.h</filename>
+          including sub-directories will be referenced from <filename>OUTFILES.c</filename>.
+        </para>
+        <para>
+          The full paths would then be <literal>$(OUTDIR)/$(dirname $OUTFILES)/$(basename $OUTFILES).{c,h}</literal>.
         </para>
       </listitem>
     </varlistentry>
@@ -230,6 +236,15 @@
       </listitem>
     </varlistentry>
 
+    <varlistentry>
+      <term><option>--output-directory</option> <replaceable>OUTDIR</replaceable></term>
+      <listitem>
+        <para>
+          Directory to output generated source to. Equivalent to changing directory before generation.
+        </para>
+      </listitem>
+    </varlistentry>
+
     <varlistentry>
       <term><option>--annotate</option> <replaceable>ELEMENT</replaceable> <replaceable>KEY</replaceable> <replaceable>VALUE</replaceable></term>
       <listitem>