Add a StaticLibrary() helper to Protoc.

This also fixes a bug where protoc couldn't be invoked multiple times on
the same inputs, because dependency files weren't shared outputs.

It's a little gross to use shared outputs here, but the alternative is
putting every .d file in its own subdir, which is even grosser.

Usage:

    cxx = builder.DetectCxx()
    protoc = builder.DetectProtoc()
    out = protoc.StaticLibrary('protos', builder, cxx, ['blah.proto'])

    other_binary = builder.Library('mylibrary')
    other_binary.compiler.sourcedeps += out.headers
    other_binary.compiler.linkflags += [out.lib.binary]
    ...

ambuild2/frontend/v2_2/tools/protoc.py

@@ -85,17 +85,18 @@ def AddSource(self, source_path):
             gen_info['sources'] += gen_source_names
             gen_info['headers'] += gen_header_names
 
-        gen_file_list += ['{}.d'.format(source_name)]
+        dep_file = '{}.d'.format(source_name)
         argv = self.argv + [
-            '--dependency_out={}'.format(gen_file_list[-1]),
+            '--dependency_out={}'.format(dep_file),
             source_path,
         ]
 
         gen_entries = self.builder.AddCommand(inputs = [source_path],
                                               argv = argv,
                                               outputs = gen_file_list,
                                               dep_type = 'md',
-                                              dep_file = gen_file_list[-1])
+                                              dep_file = dep_file,
+                                              shared_outputs = [dep_file])
 
         # Translate the list of generated output entries.
         cursor = 0
@@ -112,7 +113,12 @@ def AddSource(self, source_path):
                 self.gen_map[language].setdefault('headers', []).extend(gen_headers)
 
         # Should be one entry remaining, for the .d file.
-        assert (cursor == len(gen_entries) - 1)
+        assert (cursor == len(gen_entries))
+
+class ProtocCppNode(object):
+    def __init__(self, lib, headers):
+        self.lib = lib
+        self.headers = headers
 
 class Protoc(object):
     def __init__(self, path, name, version):
@@ -131,7 +137,7 @@ def clone(self):
 
     # Each output entry is either a language, or a tuple of (language, folder_entry).
     def Generate(self, builder, sources, outputs, includes = []):
-        runner = ProtocRunner(self, builder, includes)
+        runner = ProtocRunner(self, builder, includes + ['.'])
 
         if not outputs:
             raise Exception('No output languages were specified')
@@ -152,6 +158,16 @@ def Generate(self, builder, sources, outputs, includes = []):
 
         return runner.gen_map
 
+    def StaticLibrary(self, name, builder, cxx, sources, includes = []):
+        gen_map = self.Generate(builder, sources, ['cpp'], includes)
+        binary = cxx.StaticLibrary(name)
+        binary.compiler.sourcedeps += gen_map['cpp']['sources']
+        binary.compiler.sourcedeps += gen_map['cpp']['headers']
+        for source in gen_map['cpp']['sources']:
+            binary.sources += [os.path.join(builder.buildPath, source.path)]
+        out = builder.Add(binary)
+        return ProtocCppNode(out, gen_map['cpp']['headers'])
+
 def DetectProtoc(**kwargs):
     path = kwargs.pop('path', None)
     if len(kwargs):