never heard of something called "elif". I just know "elsif". Try that

#elif  is not the same as a combination of #else and #ifdef, but rather 
#else and #if. And #if requires an expression, which is missing if the 
macro isn't defined.

1

 #elif WITHB                           //<-- ERROR: #if with no expression

This expression equals to

1

 #else if WITHB == TRUE                          //<-- ERROR: #if with no expression

but since you only defined WITHB (without a value) it is expanded to 0, 
which equals FALSE.

Try either to define WITHB with a value or change your if-statement as 
follows:

1

 #else ifdef WITHB

Regards,

Ralf

> ... which is missing if the macro isn't defined.

Actually I meant "which is missing if the maco is defined, but empty". 
If it's not defined, that's ok.

So try:

#define WITHB 1

Wow, thats quick :-)

I tried #elsif before, but that resulted in some weired behaviour. 
Thanks to Ralf and Rolf's solution it works as intended now!

Thanks very much for your help!

1

#define WITHB 1