Change to disable a warning for GCC compilation

The warning was only shown and asked not to be treated as an error.
However, some recent flag change of chromeos toolchain has some
interference with the warning. To avoid future problem, disable the
warning for GCC.

BUG=chromium:615424

Review-Url: https://codereview.chromium.org/2013163003
diff --git a/BUILD.gn b/BUILD.gn
index 2d75349..1550209 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -51,7 +51,7 @@
   }
 
   if (is_posix && !is_clang) {  # When GCC.
-    cflags += [ "-Wno-error=strict-overflow" ]
+    cflags += [ "-Wno-strict-overflow" ]
   }
 }
 
diff --git a/xfa.gyp b/xfa.gyp
index ead79d3..14a8130 100644
--- a/xfa.gyp
+++ b/xfa.gyp
@@ -699,7 +699,7 @@
         ["clang==1" , {
         }],
         ["os_posix==1 and clang==0", { # When GCC
-          'cflags': [ '-Wno-error=strict-overflow' ],
+          'cflags': [ '-Wno-strict-overflow' ],
         }],
         ["pdf_enable_v8==1", {
           'dependencies': [