Define new macro to point to extern dir.

project/vc8/nvtt.vsprops

@@ -12,16 +12,20 @@
 		Name="SrcDir"
 		Value="$(SolutionDir)..\..\src"
 	/>
+	<UserMacro
+		Name="ExternDir"
+		Value="$(SolutionDir)..\..\extern"
+	/>
 	<UserMacro
 		Name="PoshDir"
-		Value="$(SolutionDir)..\..\extern\poshlib"
+		Value="$(ExternDir)\poshlib"
 	/>
 	<UserMacro
 		Name="GnuWinDir"
-		Value="$(SolutionDir)..\..\extern\gnuwin32"
+		Value="$(ExternDir)\gnuwin32"
 	/>
 	<UserMacro
 		Name="FreeImageDir"
-		Value="$(SolutionDir)..\..\extern\FreeImage"
+		Value="$(ExternDir)\FreeImage"
 	/>
 </VisualStudioPropertySheet>