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

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You can simulate conditionals using macro token concatenation as follows:

#define DEF_CONST(b_const) DEF_CONST_##b_const
#define DEF_CONST_true const
#define DEF_CONST_false

Then,

/* OK */
DEF_CONST(true)  int x;  /* expands to const int x */
DEF_CONST(false) int y;  /* expands to int y */

/* NOT OK */
bool bSomeBool = true;       // technically not C :)
DEF_CONST(bSomeBool) int z;  /* error: preprocessor does not know the value
                                of bSomeBool */

Also, allowing for passing macro parameters to DEF_CONST itself (as correctly pointed out by GMan and others):

#define DEF_CONST2(b_const) DEF_CONST_##b_const
#define DEF_CONST(b_const) DEF_CONST2(b_const)
#define DEF_CONST_true const
#define DEF_CONST_false

#define b true
#define c false

/* OK */
DEF_CONST(b) int x;     /* expands to const int x */
DEF_CONST(c) int y;     /* expands to int y */
DEF_CONST(true) int z;  /* expands to const int z */

You may also consider the much simpler (though potentially less flexible):

#if b_const
# define DEF_CONST const
#else /*b_const*/
# define DEF_CONST
#endif /*b_const*/

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