$ cat foo.c
#include <stdio.h>
#include <stdlib.h>

#if defined(DO_INT)
int     a1[] = { 1, 100, 5, 50, 25 };
#endif

#if defined(DO_DOUBLE)
double  a2[] = { 1.0, 10.0, 5.0, 25.0 };
#endif

int (*cmp_fn)(const void *, const void *);

#define CFG_ADD(TYPE)       \
    extern int compare_ ## TYPE(const void*, const void*);   \
    cmp_fn = compare_ ## TYPE

int main(int argc, char **argv) {

#if defined(DO_INT)
    CFG_ADD(int);
    qsort(a1, sizeof(a1)/sizeof(a1[0]), sizeof(a1[0]), cmp_fn);
    for (int i=0; i<sizeof(a1)/sizeof(a1[0]); i++) {
        printf("%d, ", a1[i]);
    }
#endif

#if defined(DO_DOUBLE)
    printf("\n\n");
    CFG_ADD(double);
    qsort(a2, sizeof(a2)/sizeof(a2[0]), sizeof(a2[0]), cmp_fn);
    for (int i=0; i<sizeof(a2)/sizeof(a2[0]); i++) {
        printf("%f, ", a2[i]);
    }
#endif

}

$ gcc foo.c -DDO_INT -L. -ltypes -o foo_int

$ gcc foo.c -DDO_DOUBLE -L. -ltypes -o foo_double

$ gcc foo.c -DDO_INT -DDO_DOUBLE -L. -ltypes -o foo_both