Fixed compile- and link-time errors with the "enable_vulkan" GN-argument

gpu/vulkan/BUILD.gn

Index: gpu/vulkan/BUILD.gn
diff --git a/gpu/vulkan/BUILD.gn b/gpu/vulkan/BUILD.gn
index 758a14ba7f9e156390bd098812653c071912f365..39ed1e066822ed9cf657bb46ecc84e2de310d77d 100644
--- a/gpu/vulkan/BUILD.gn
+++ b/gpu/vulkan/BUILD.gn
@@ -10,6 +10,7 @@ config("vulkan_config") {
 }
 
 if (enable_vulkan) {
+  vulkan_lib_dir = getenv("VULKAN_SDK") + "/lib"

[piman, 2016/11/01 19:45:13]

What is this for? The expectation is that the vulkan run-time is installed in
the normal location (/usr/lib/x86_64-linux-gnu) as defined by the package.

[Sergey.Kipet, 2016/11/04 22:15:36]

Well, seems like starting from the SDK version 1.0.13, it is not assumed as
location for installing Vulkan run-time anymore (please, refer to the
'Administrative privileges' section at
https://vulkan.lunarg.com/doc/sdk/latest/linux/getting_started.html).
Particularly, the 'libvulkan_wrapper.so' sub-target is successfully linked if
that kind of workaround is provided while having the most recent SDK installed
only.

[piman, 2016/11/05 00:39:06]

Ok. This is not a great long-term situation, but because it's all behind a flag,
no need to block on it.

[Sergey.Kipet, 2016/11/05 09:10:11]

Acknowledged.

   component("vulkan") {
     output_name = "vulkan_wrapper"
 
@@ -56,7 +57,10 @@ if (enable_vulkan) {
       libs = [ "vulkan" ]
 
       if (current_cpu == "x64") {
-        lib_dirs = [ "/usr/lib/x86_64-linux-gnu" ]
+        lib_dirs = [
+          "/usr/lib/x86_64-linux-gnu",
+          vulkan_lib_dir,
+        ]
       } else {
         assert(false, "Unsupported vulkan target: " + current_cpu)
       }
@@ -87,7 +91,10 @@ if (enable_vulkan) {
 
     include_dirs = [ "/usr/include" ]
     if (current_cpu == "x64") {
-      lib_dirs = [ "/usr/lib/x86_64-linux-gnu" ]
+      lib_dirs = [
+        "/usr/lib/x86_64-linux-gnu",
+        vulkan_lib_dir,
+      ]
     } else {
       assert(false, "Unsupported vulkan target: " + current_cpu)
     }