[Vala] Conditionally compiling from FAQ

[Brooss <brooss teambb gmail com>]

In the 'Does Vala have a preprocessor?' section of the FAQ page a small 
example is given on how to conditionally compile Vala code but I don't 
seem to be able to get it to work.
The FAQ says I should add '-D COND' to the valac command line but doing 
so just tells me 'unknown option -D', has the syntax changed or am I 
reading the example wrong?
Thanks.