Go to your Target or Project settings, click the Gear icon at the bottom left, and select "Add User-Defined Setting". The new setting name should be
GCC_PREPROCESSOR_DEFINITIONS, and you can type your definitions in the right-hand field.
Per Steph's comments, the full syntax is:
Note that you don't need the '='s if you just want to #define a symbol, rather than giving it a value (for