How to use #if inside #define in the C preprocessor?

Question

I want to write a macro that spits out code based on the boolean value of its parameter. So say DEF_CONST(true) should be expanded into const, and

Answer 1

Doing it as a paramterised macro is a bit odd.

Why not just do something like this:

#ifdef USE_CONST
    #define MYCONST const
#else
    #define MYCONST
#endif

Then you can write code like this:

MYCONST int x = 1;
MYCONST char* foo = "bar";

and if you compile with USE_CONST defined (e.g. typically something -DUSE_CONST in the makefile or compiler options) then it will use the consts, otherwise it won't.

Edit: Actually I see Vlad covered that option at the end of his answer, so +1 for him :)