gdbus-codegen: Add support for pragma inclusion guard

The #pragma once is widely supported preprocessor directive that can
be used instead of include guards.

This adds support for using optionally this directive instead of
include guards.

https://bugzilla.gnome.org/show_bug.cgi?id=791015

docs/reference/gio/gdbus-codegen.xml

@@ -35,6 +35,7 @@
     <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>--pragma-once</option></arg>
     <arg><option>--xml-files</option> <replaceable>FILE</replaceable></arg>
     <group choice="plain" rep="repeat">
       <arg>
@@ -215,6 +216,17 @@
       </listitem>
     </varlistentry>
 
+    <varlistentry>
+      <term><option>--pragma-once</option></term>
+      <listitem>
+        <para>
+          If this option is passed, the
+          <ulink url="https://en.wikipedia.org/wiki/Pragma_once">&#35;pragma once</ulink>
+          preprocessor directive is used instead of include guards.
+        </para>
+      </listitem>
+    </varlistentry>
+
     <varlistentry>
       <term><option>--c-generate-object-manager</option></term>
       <listitem>

gio/gdbus-2.0/codegen/codegen.py

@@ -37,7 +37,8 @@ LICENSE_STR = '''/*
 
 class CodeGenerator:
     def __init__(self, ifaces, namespace, interface_prefix, generate_objmanager,
-                 generate_autocleanup, docbook_gen, h, c, header_name):
+                 generate_autocleanup, docbook_gen, h, c, header_name,
+                 use_pragma):
         self.docbook_gen = docbook_gen
         self.generate_objmanager = generate_objmanager
         self.generate_autocleanup = generate_autocleanup
@@ -59,6 +60,7 @@ class CodeGenerator:
             self.ns_lower = ''
         self.interface_prefix = interface_prefix
         self.header_guard = header_name.upper().replace('.', '_').replace('-', '_').replace('/', '_').replace(':', '_')
+        self.use_pragma = use_pragma
 
     # ----------------------------------------------------------------------------------------------------
 
@@ -224,8 +226,13 @@ class CodeGenerator:
 
         self.h.write(LICENSE_STR.format(config.VERSION))
         self.h.write('\n')
-        self.h.write('#ifndef __{!s}__\n'.format(self.header_guard))
-        self.h.write('#define __{!s}__\n'.format(self.header_guard))
+
+        if self.use_pragma:
+            self.h.write('#pragma once\n')
+        else:
+            self.h.write('#ifndef __{!s}__\n'.format(self.header_guard))
+            self.h.write('#define __{!s}__\n'.format(self.header_guard))
+
         self.h.write('\n')
         self.h.write('#include <gio/gio.h>\n')
         self.h.write('\n')
@@ -746,10 +753,12 @@ class CodeGenerator:
     # ----------------------------------------------------------------------------------------------------
 
     def generate_outro(self):
-        self.h.write('\n'
-                     'G_END_DECLS\n'
-                     '\n'
-                     '#endif /* __%s__ */\n'%(self.header_guard))
+        self.h.write('\n')
+        self.h.write('G_END_DECLS\n')
+
+        if not self.use_pragma:
+            self.h.write('\n')
+            self.h.write('#endif /* __{!s}__ */\n'.format(self.header_guard))
 
     # ----------------------------------------------------------------------------------------------------
 

gio/gdbus-2.0/codegen/codegen_main.py

@@ -163,6 +163,8 @@ def codegen_main():
                             help='Generate autocleanup support')
     arg_parser.add_argument('--generate-docbook', metavar='OUTFILES',
                             help='Generate Docbook in OUTFILES-org.Project.IFace.xml')
+    arg_parser.add_argument('--pragma-once', action='store_true',
+                            help='Use "pragma once" as the inclusion guard')
     arg_parser.add_argument('--annotate', nargs=3, action='append', metavar='WHAT KEY VALUE',
                             help='Add annotation (may be used several times)')
     arg_parser.add_argument('--output-directory', metavar='OUTDIR', default='',
@@ -204,7 +206,8 @@ def codegen_main():
                                     args.c_generate_autocleanup,
                                     docbook_gen,
                                     h, c,
-                                    header_name)
+                                    header_name,
+                                    args.pragma_once)
         ret = gen.generate()
         h.close()
         c.close()