Fix GCC build

BUILD.gn

 # Copyright 2016 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.
 
 import("//build_overrides/v8.gni")
 import("//testing/test.gni")
 import("pdfium.gni")
 
 config("pdfium_config") {
   cflags = []
@@ skipping 47 matching lines @@
       "/wd4245",
       "/wd4310",
       "/wd4389",
       "/wd4701",
       "/wd4702",
       "/wd4706",
       "/wd4800",
     ]
   }
 
-  if (is_clang) {
+  if (is_clang || is_posix) {
     # TODO(thestig): Fix all instances, remove this, pdfium:29
     cflags += [ "-Wno-sign-compare" ]
   }
+
+  if (is_posix && !is_clang) {  # When GCC.
+    cflags += [ "-Wno-error=strict-overflow" ]
+  }
 }
 
 static_library("pdfium") {
   sources = [
     "fpdfsdk/include/fsdk_actionhandler.h",
     "fpdfsdk/include/fsdk_annothandler.h",
     "fpdfsdk/include/fsdk_baseannot.h",
     "fpdfsdk/include/fsdk_baseform.h",
     "fpdfsdk/src/fpdf_dataavail.cpp",
     "fpdfsdk/src/fpdf_ext.cpp",
@@ skipping 1441 matching lines @@
       "//v8:v8_libplatform",
     ]
     include_dirs += [
       "//v8",
       "//v8/include",
     ]
     configs += [ "//v8:external_startup_data" ]
   }
   configs += [ ":pdfium_config" ]
 }