Innosetup #include not working from build script

[wellilein]

I have a Visual Build script which builds my Innosetup installer (Setup.iss). Recently I added a backup/restore functionality which is realized as an inclusion of two additional ISS files using the preprocessor:
[code]
[Files]
#include <BackupFiles.iss>
...

procedure ...
#include <RestoreFiles.iss>
end;

I can build the Setup.iss manually by opening the file and compiling. However, I can not build the file with Visual Build. The error message is:

Inno Setup 5 Command-Line Compiler
Copyright (C) 1997-2009 Jordan Russell. All rights reserved.
Portions by Martijn Laan
Inno Setup Preprocessor
Copyright (C) 2001-2002 Alex Yackimoff. All rights reserved.
Compiler engine version: Inno Setup 5.3.9 (u) - ISPP 5.3.9 (u)
[ISPP] Preprocessing.
Error on line 61 in P:\...\Setup.iss: [ISPP] File not found: "BackupFiles.iss".
Compile aborted.
Process completed with exit code 2

I am using InnoSetup 5.3.9 (u), preprocessor 5.3.9 (u) and Visual Build 6.7a. It might be a working directory issue, because the build script runs from a different location compared to manually compiling the Setup. But how can I set the working directory for the InnoSetup action?