Thanks for your answers.
Robtillart: if the configuration file is in the library folder, it's OK. I can compile and run the application
but if the configuration file is in the sketch folder, following message is displayed:
"D:\arduino-1.0\libraries\MasterSlaveLib\MasterSlaveLib.cpp:34:29: error: MasterSlaveLib_Conf.h: No such file or directory"
My goal is to minimize the code generated regarding sketch configuration (e.g. application shall support all or some Master or Slave or both Master and Slave functions) that's why I need to have some #define.
To solve my problem, II know I can duplicate the library source files in all sketch folders and it will be OK but I loose the advantage to have only one source for all my projects.
I can also split my library to several part but maintenance will become very difficult.
OK, I'll continue to investigate