third-party.gni: Search includes using -I.

This will ensure that the headers from the dependencies will have
precedent over system headers, thus preventing situations where system
headers will block dependency headers and prevent compilation.

Change-Id: I0d480a6d3898f2da99cf2706c5335aaac05b4e4d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/220276
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>

AUTHORS

@@ -40,6 +40,7 @@ Raul Tambre <raul@tambre.ee>
 Samsung <*@samsung.com>
 Samsung Open Source Group <*@osg.samsung.com>
 Sergey Melnikov <Melnikov.Sergey.V@gmail.com>
+Shachar Langbeheim <nihohit@gmail.com>
 Skia <*@skia.org>
 Skia Buildbots <skia.buildbots@gmail.com>
 Sony Mobile Communications Inc. <*@sonymobile.com>

gn/BUILD.gn

@@ -11,7 +11,6 @@ declare_args() {
   extra_ldflags = []
 
   malloc = ""
-  werror = false
 }
 
 if (is_ios) {

gn/BUILDCONFIG.gn

@@ -34,6 +34,7 @@ declare_args() {
   clang_win = ""
 
   skia_moltenvk_path = ""
+  werror = false
 }
 declare_args() {
   is_debug = !is_official_build

third_party/third_party.gni

@@ -23,10 +23,17 @@ template("third_party") {
         }
       } else {
         foreach(dir, invoker.public_include_dirs) {
-          cflags += [
-            "-isystem",
-            rebase_path(dir),
-          ]
+          if (werror) {
+            cflags += [
+              "-isystem",
+              rebase_path(dir),
+            ]
+          } else {
+            cflags += [
+              "-I",
+              rebase_path(dir),
+            ]
+          }
         }
       }
     } else {