Okay, I hacked it out.
The build machine (WinXP) has a Windows SDK 7.1 with AL.exe 10.0.30.x. This is the one always creating .NET4-satellite assemblies.
My machine (Win7) with VS2010 has a Windows SDK 7.0a with AL.exe 9.0.30729.1. This is the one respecting the target framework.
I have (after backup) replace the AL.exe from the build machine with the one from mine. This works but I consider it a very dirty workaround.
If anyone knows a better way I would appreciate any help!