Added the ability to specify the glfw vulkan library. (#731)

Requirement for #42225

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Author: gaaclarke

build/secondary/third_party/glfw/BUILD.gn

@@ -2,6 +2,8 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("glfw_args.gni")
+
 _checkout_dir = "//third_party/glfw"
 
 config("relative_glfw_headers") {
@@ -117,6 +119,10 @@ source_set("glfw") {
     ]
   }
 
+  if (glfw_vulkan_library != "") {
+    defines += [ "_GLFW_VULKAN_LIBRARY=" + glfw_vulkan_library ]
+  }
+
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ "//build/config/compiler:no_chromium_code" ]
 }

build/secondary/third_party/glfw/glfw_args.gni

@@ -0,0 +1,7 @@
+# Copyright 2013 The Flutter Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+declare_args() {
+  glfw_vulkan_library=""
+}