Q:

Change Users Variables using compiler directive? 

How can I change a user variable like $(Test) via code, using a compiler directive or via project options? 
I have defined this variable that sometimes points to a trunk svn source code and sometimes it points to a tag svn source.  I use this var in Library Path.  When I open the trunk project i need the var to be set to the trunk svn source and when i open the tag svn project i need the var to be set to the tag svn.  Both projects in the trunk and tag are the same, but the trunk has some modifications (it is the code i am working at and it is not ready to be used yet). 
I do not want to go to the Tools options to set it every time i change projects. 
EDIT:
I know I could use the project source path instead of the library path, but I need all sources to be on library path so I can easily debug a project that is using my framework. 

A:

I'd probably create two batch files.  Each would first set the Test environmental variable to the necessary value, then start the IDE. 
And then every time I needed to work with a different set of svn source files I'd have to (re)start the IDE through the appropriate batch file.