[gtkmm] NMake Makefiles: Add default Vulkan SDK path

[Chun-wei Fan <fanchunwei src gnome org>]

commit 8fb5e4b04ed3f3ba4dd0c5623be11b7841c46c1b
Author: Chun-wei Fan <fanchunwei src gnome org>
Date:   Thu Mar 12 19:00:13 2020 +0800

    NMake Makefiles: Add default Vulkan SDK path
    
    The LunarG Vulkan SDK installer adds an envvar VULKAN_SDK to the system,
    so we can use it as a default PATH for finding the headers and libraries
    for the Vulkan SDK.

 MSVC_NMake/config-msvc.mak | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/MSVC_NMake/config-msvc.mak b/MSVC_NMake/config-msvc.mak
index 30abbf85..62385f40 100644
--- a/MSVC_NMake/config-msvc.mak
+++ b/MSVC_NMake/config-msvc.mak
@@ -7,6 +7,7 @@
 !message normally c:\VulkanSDK\$$(YOUR_LUNARG_VULKAN_SDK_VER) to the
 !message NMake command line, and retry the build.
 !message
+VULKAN_SDK_DIR=$(VULKAN_SDK)
 !endif
 
 # These are the base minimum libraries required for building gtkmm.