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

gio/gdbus-2.0/codegen/codegen_main.py

@@ -21,6 +21,7 @@
 
 import sys
 import optparse
+from os import path
 
 from . import config
 from . import utils
@@ -162,6 +163,8 @@ def codegen_main():
                           help='Generate Docbook in OUTFILES-org.Project.IFace.xml')
     arg_parser.add_option('', '--annotate', nargs=3, action='append', metavar='WHAT KEY VALUE',
                           help='Add annotation (may be used several times)')
+    arg_parser.add_option('', '--output-directory', metavar='OUTDIR', default='',
+                          help='Location to output generated files')
     (opts, args) = arg_parser.parse_args();
 
     all_ifaces = []
@@ -185,15 +188,18 @@ def codegen_main():
 
     c_code = opts.generate_c_code
     if c_code:
-        h = open(c_code + '.h', 'w')
-        c = open(c_code + '.c', 'w')
+        outdir = opts.output_directory
+        header_name = c_code + '.h'
+        h = open(path.join(outdir, header_name), 'w')
+        c = open(path.join(outdir, c_code + '.c'), 'w')
         gen = codegen.CodeGenerator(all_ifaces,
                                     opts.c_namespace,
                                     opts.interface_prefix,
                                     opts.c_generate_object_manager,
                                     opts.c_generate_autocleanup,
                                     docbook_gen,
-                                    h, c);
+                                    h, c,
+                                    header_name)
         ret = gen.generate()
         h.close()
         c.close()