gdbus-codegen: Apply --output-directory to generated docs as well

In addition to code, gdbus-codegen can also generate docbook
documentation for DBus interfaces. There's no good reason why
the newly added --output-directory option shouldn't apply to
those generated files as well.

https://bugzilla.gnome.org/show_bug.cgi?id=783201

gio/gdbus-2.0/codegen/codegen_docbook.py

@@ -21,6 +21,7 @@
 
 import sys
 import re
+from os import path
 
 from . import config
 from . import utils
@@ -30,9 +31,10 @@ from . import parser
 # ----------------------------------------------------------------------------------------------------
 
 class DocbookCodeGenerator:
-    def __init__(self, ifaces, docbook):
+    def __init__(self, ifaces, docbook, outdir):
         self.ifaces = ifaces
         self.docbook = docbook
+        self.outdir = outdir
         self.generate_expand_dicts()
 
     def print_method_prototype(self, i, m, in_synopsis):
@@ -270,7 +272,7 @@ class DocbookCodeGenerator:
 
     def generate(self):
         for i in self.ifaces:
-            self.out = open('%s-%s.xml'%(self.docbook, i.name), 'w')
+            self.out = open(path.join(self.outdir, '%s-%s.xml'%(self.docbook, i.name)), 'w')
             self.out.write(''%())
             self.out.write('<?xml version="1.0" encoding="utf-8"?>\n'%())
             self.out.write('<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"\n'%())