Introduce a user macro named $(externalsDir), which should point to the root…

Introduce a user macro named $(externalsDir), which should point to the root directory of where all the external sources should live.  Developers can change this value if their external sources live elsewhere.  The default of '..\..' matches the current status quo.

PCbuild/pyproject.vsprops

@@ -44,6 +44,10 @@
 		Name="PythonExe"
 		Value="$(SolutionDir)\python.exe"
 	/>
+	<UserMacro
+		Name="externalsDir"
+		Value="..\.."
+	/>
 	<UserMacro
 		Name="bsddbDir"
 		Value="$(bsddb44Dir)"
@@ -54,7 +58,7 @@
 	/>
 	<UserMacro
 		Name="bsddb44Dir"
-		Value="..\..\db-4.4.20\build_win32"
+		Value="$(externalsDir)\db-4.4.20\build_win32"
 	/>
 	<UserMacro
 		Name="bsddb44DepLibs"
@@ -62,7 +66,7 @@
 	/>
 	<UserMacro
 		Name="bsddb45Dir"
-		Value="..\..\db-4.5.20.x\build_windows"
+		Value="$(externalsDir)\db-4.5.20.x\build_windows"
 	/>
 	<UserMacro
 		Name="bsddb45DepLibs"
@@ -70,23 +74,23 @@
 	/>
 	<UserMacro
 		Name="sqlite3Dir"
-		Value="..\..\sqlite-source-3.3.4"
+		Value="$(externalsDir)\sqlite-source-3.3.4"
 	/>
 	<UserMacro
 		Name="bz2Dir"
-		Value="..\..\bzip2-1.0.3"
+		Value="$(externalsDir)\bzip2-1.0.3"
 	/>
 	<UserMacro
 		Name="opensslDir"
-		Value="..\..\openssl-0.9.8g"
+		Value="$(externalsDir)\openssl-0.9.8g"
 	/>
 	<UserMacro
 		Name="tcltkDir"
-		Value="..\..\tcltk"
+		Value="$(externalsDir)\tcltk"
 	/>
 	<UserMacro
 		Name="tcltk64Dir"
-		Value="..\..\tcltk64"
+		Value="$(externalsDir)\tcltk64"
 	/>
 	<UserMacro
 		Name="tcltkLib"