#define CFG_ADD(TYPE,NAME,STUFF) \
    #ifndef USING_ ## TYPE /* does not work */ \
    #define USING_ ## TYPE /* does not work */ \
    #endif
#include "configfile.h"
#undef CFG_ADD

/* and later: */

#ifdef USING_int
static int compare_int(const void * a, const void * b);
#endif
#ifdef USING_double
static int compare_double(const void * a, const void * b);
#endif

/* further down */

#ifdef USING_int
static int compare_int(const void * a, const void * b)
{
    /* ... */
}
#endif

#ifdef USING_double
static int compare_double(const void * a, const void * b)
{
    /* ... */
}
#endif