Allow buildroot to specify location of vulkan_memory_allocator

Change-Id: I6dab23fbd4d663e4031e3c6f568471f266f26936
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/547016
Reviewed-by: Ben Wagner <bungeman@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>

gn/skia.gni

@@ -131,6 +131,8 @@ declare_args() {
   skia_enable_direct3d_debug_layer = skia_enable_gpu_debug_layers
   skia_enable_metal_debug_info = skia_enable_gpu_debug_layers
   skia_use_vma = skia_use_vulkan
+  skia_vulkan_memory_allocator_dir =
+      "//third_party/externals/vulkanmemoryallocator"
   skia_build_for_debugger = false
 }
 

third_party/vulkanmemoryallocator/BUILD.gn

@@ -8,7 +8,7 @@ import("../../gn/skia.gni")
 config("vulkanmemoryallocator_public") {
   include_dirs = [
     ".",
-    "//third_party/externals/vulkanmemoryallocator/include",
+    "$skia_vulkan_memory_allocator_dir/include",
   ]
 }