Linux: add an option to build with system protobuf.

build/protoc.gypi

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # This file is meant to be included into a target to provide a rule
 # to invoke protoc in a consistent manner.
 #
 # To use this, create a gyp target with the following form:
 # {
 #   'target_name': 'my_proto_lib',
@@ skipping 30 matching lines @@
 # e.g. 'base/base_export.h'
 #
 # Implementation notes:
 # A proto_out_dir of foo/bar produces
 #   <(SHARED_INTERMEDIATE_DIR)/protoc_out/foo/bar/{file1,file2}.pb.{cc,h}
 #   <(SHARED_INTERMEDIATE_DIR)/pyproto/foo/bar/{file1,file2}_pb2.py
 
 {
   'variables': {
     'protoc_wrapper': '<(DEPTH)/tools/protoc_wrapper/protoc_wrapper.py',
-    'protoc': '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)protoc<(EXECUTABLE_SUFFIX)',
     'cc_dir': '<(SHARED_INTERMEDIATE_DIR)/protoc_out/<(proto_out_dir)',
     'py_dir': '<(PRODUCT_DIR)/pyproto/<(proto_out_dir)',
     'cc_generator_options%': '',
     'cc_include%': '',
     'proto_in_dir%': '.',
+    'conditions': [
+      ['use_system_protobuf==0', {
+        'protoc': '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)protoc<(EXECUTABLE_SUFFIX)',
+      }, { # use_system_protobuf==1
+        'protoc': '<!(which protoc)',
+      }],
+    ],
   },
   'rules': [
     {
       'rule_name': 'genproto',
       'extension': 'proto',
       'inputs': [
         '<(protoc_wrapper)',
         '<(protoc)',
       ],
       'outputs': [
@@ skipping 40 matching lines @@
   },
   'export_dependent_settings': [
     # The generated headers reference headers within protobuf_lite,
     # so dependencies must be able to find those headers too.
     '<(DEPTH)/third_party/protobuf/protobuf.gyp:protobuf_lite',
   ],
   # This target exports a hard dependency because it generates header
   # files.
   'hard_dependency': 1,
 }