Xcode environment variables for sub projects
you accomplish this without multiple definitions by creating xcconfig files and then referencing or #include-ing them throughout your projects. so, you could apply Mattias' suggestion and then define the preprocessor defs in the xcconfig. then you have one file to change (and a full rebuild if you require these defs in the pch file, which there are separate settings for).
xcode also lets you assign separate xcconfigs per build configuration.
I would edit the schema and add a pre-build shell script to set the proper variables.When you add a script you can establish from which target you are getting the definitions.
Maybe add a define using the "Preprocessor macros" build setting to the targets and or debug/release build configurations where you want to enable debug.
In your case you would double click on on the value column and then click "+" to add a new macro. The marco would be "FOOMODULE_DEBUG_LEVEL=1" which should result in -DFOOMODULE_DEBUG_LEVEL=1 to the compiler.