basic support for 'build_always' feature of custom targets

build.py

@@ -667,7 +667,8 @@ class CustomTarget:
                     'output' : True,
                     'command' : True,
                     'install' : True,
-                    'install_dir' : True}
+                    'install_dir' : True,
+                    'build_always' : True}
     def __init__(self, name, subdir, kwargs):
         self.name = name
         self.subdir = subdir
@@ -740,6 +741,7 @@ class CustomTarget:
                 raise InvalidArguments('"install_dir" must be a string.')
         else:
             self.install = False
+        self.build_always = kwargs.get('build_always', False)
 
     def get_basename(self):
         return self.name

ninjabackend.py

@@ -281,6 +281,8 @@ class NinjaBackend(backends.Backend):
         deps = [os.path.join(i.get_subdir(), self.hackety_hack(i.get_filename())) for i in target.get_dependencies()]
         srcs = [os.path.join(self.build_to_src, target.subdir, i) for i in target.sources]
         deps +=  srcs
+        if target.build_always:
+            deps.append('PHONY')
         elem = NinjaBuildElement(ofilenames, 'CUSTOM_COMMAND', deps)
         cmd = []
         for i in target.command: