Allow Mac precompiled headers to be disabled based on a GYP variable.

build/common.gypi

 # Copyright (c) 2009 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.
 
 {
   'variables': {
+    # .gyp files should set chromium_code to 1 if they build Chromium-specific
+    # code, as opposed to external code.  This variable is used to control
+    # such things as the set of warnings to enable, and whether warnings are
+    # treated as errors.
     'chromium_code%': 0,
+
+    # Variables expected to be overriden on the GYP command line (-D) or by
+    # ~/.gyp/include.gypi.
+
+    # Override chromium_mac_pch and set it to 0 to suppress the use of
+    # precompiled headers on the Mac.  Prefix header injection may still be
+    # used, but prefix headers will not be precompiled.  This is useful when
+    # using distcc to distribute a build to compile slaves that don't
+    # share the same compiler executable as the system driving the compilation,
+    # because precompiled headers rely on pointers into a specific compiler
+    # executable's image.  Setting this to 0 is needed to use an experimental
+    # Linux-Mac cross compiler distcc farm.
+    'chromium_mac_pch%': 1,
+
+    # Override branding to select the desired branding flavor.
     'branding%': 'Chromium',
   },
   'target_defaults': {
     'conditions': [
       ['branding=="Chrome"', {
         'defines': ['GOOGLE_CHROME_BUILD'],
       }, {  # else: branding!="Chrome"
         'defines': ['CHROMIUM_BUILD'],
       }],
     ],
@@ skipping 52 matching lines @@
   'conditions': [
     ['OS=="mac"', {
       'target_defaults': {
         'xcode_settings': {
           'ALWAYS_SEARCH_USER_PATHS': 'NO',
           'GCC_C_LANGUAGE_STANDARD': 'c99',
           'GCC_CW_ASM_SYNTAX': 'NO',
           'GCC_DYNAMIC_NO_PIC': 'YES',
           'GCC_ENABLE_PASCAL_STRINGS': 'NO',
           'GCC_INLINES_ARE_PRIVATE_EXTERN': 'YES',
-          'GCC_PRECOMPILE_PREFIX_HEADER': 'YES',
           'GCC_SYMBOLS_PRIVATE_EXTERN': 'YES',
           'GCC_TREAT_WARNINGS_AS_ERRORS': 'YES',
           'GCC_VERSION': '4.2',
           'GCC_WARN_ABOUT_MISSING_NEWLINE': 'YES',
           'MACOSX_DEPLOYMENT_TARGET': '10.5',
           'PREBINDING': 'NO',
           'SDKROOT': 'macosx10.5',
           'USE_HEADERMAP': 'NO',
           'WARNING_CFLAGS': ['-Wall', '-Wendif-labels'],
+          'conditions': [
+            ['chromium_mac_pch', {'GCC_PRECOMPILE_PREFIX_HEADER': 'YES'},
+                                 {'GCC_PRECOMPILE_PREFIX_HEADER': 'NO'}],
+          ],
         },
         'target_conditions': [
           ['_type=="shared_library"', {
             'xcode_settings': {'GCC_DYNAMIC_NO_PIC': 'NO'},
           }],
           ['_type!="static_library"', {
             'xcode_settings': {'OTHER_LDFLAGS': ['-Wl,-search_paths_first']},
           }],
           ['_type=="application"', {
             'xcode_settings': {'OTHER_LDFLAGS': ['-Wl,-ObjC']},
@@ skipping 105 matching lines @@
     # settings in target dicts.  SYMROOT is a special case, because many other
     # Xcode variables depend on it, including variables such as
     # PROJECT_DERIVED_FILE_DIR.  When a source group corresponding to something
     # like PROJECT_DERIVED_FILE_DIR is added to a project, in order for the
     # files to appear (when present) in the UI as actual files and not red
     # red "missing file" proxies, the correct path to PROJECT_DERIVED_FILE_DIR,
     # and therefore SYMROOT, needs to be set at the project level.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }