Silence shift-negative-value warnings only on POSIX/Clang.

third_party/BUILD.gn

 # Copyright 2014 PDFium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("../pdfium.gni")
 
 group("third_party") {
   deps = [
     ":bigint",
     ":fx_freetype",
@@ skipping 161 matching lines @@
     "lcms2-2.6/src/cmstypes.c",
     "lcms2-2.6/src/cmsvirt.c",
     "lcms2-2.6/src/cmswtpnt.c",
     "lcms2-2.6/src/cmsxform.c",
   ]
 }
 
 # This is only used for standalone builds.
 config("jpeg_warnings") {
   visibility = [ ":*" ]
-  cflags = [ "-Wno-shift-negative-value" ]
+  if (is_clang) {
+    cflags = [ "-Wno-shift-negative-value" ]
+  }
 }
 
 source_set("jpeg") {
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [
     "//build/config/compiler:no_chromium_code",
     "//third_party/pdfium:pdfium_config",
 
     # Must be after no_chromium_code for warning flags to be ordered correctly.
     ":jpeg_warnings",
@@ skipping 164 matching lines @@
       "libtiff/tif_warning.c",
       "libtiff/tif_write.c",
       "libtiff/tif_zip.c",
     ]
   }
 }
 
 # TODO(dsinclair): Remove if fixed upstream. https://crbug.com/507712
 config("fx_zlib_warnings") {
   visibility = [ ":*" ]
-  cflags = [ "-Wno-shift-negative-value" ]
+  if (is_clang) {
+    cflags = [ "-Wno-shift-negative-value" ]
+  }
 }
 
 source_set("fx_zlib") {
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [
     "//build/config/compiler:no_chromium_code",
     "//third_party/pdfium:pdfium_config",
 
     # Must be after no_chromium_code for warning flags to be ordered correctly.
     ":fx_zlib_warnings",
@@ skipping 26 matching lines @@
   sources = [
     "base/logging.h",
     "base/macros.h",
     "base/numerics/safe_conversions.h",
     "base/numerics/safe_conversions_impl.h",
     "base/numerics/safe_math.h",
     "base/numerics/safe_math_impl.h",
     "base/stl_util.h",
   ]
 }