tinylog.c, log2 calculation for both FPU and no FPU

tinylog.c

/* Simple log2 calculation; on FPU-less platform it will bitshift instead of log2 */
//
// linux/mac:
// clang -fdiagnostics-color=always -g ./tinylog.c -o ./tinylog
// CC65:
// cl65 -t atari ./tinylog.c  -o tinylog.xex
// atari800 -nobasic -run ./tinylog.xex

#include<stdio.h>
#include<stdint.h>

// This isn't very portable because __CC65__ isn't the only FPU-less platform.
// Many embedded microcontrollers define __FPU_PRESENT 0, so the question is should CC65 also define __FPU_PRESENT?
#if defined (__CC65__)
 #include<cc65.h>
 #include<conio.h>
 #warning "CC65 detected, so avoiding float operations."
#else
 #include<math.h>
#endif /* __CC65__ */  

int main(void)
{
    uint16_t input;     // number (range) limit, user-defined
    uint16_t output;
    output = 0;

    printf("Input number you want log2 of:");
    scanf("%hu", &input);
    printf("You input: %d\n", input);

    // get log2

    // ... basically if we had _FPU_PRESENT we could just do:
    /// 
    #if defined (__CC65__)
    while (input) {
        input >>= 1;
        output ++;
    }
    #else
    output = (int)(log(input) / log(2))+1;
    #endif /* __CC65__ */

    printf("log2 result:%d\n", output);

    // For platforms that clear the screen on exit, wait for user input
    #if defined (__CC65__)
      if (doesclrscrafterexit()){
            printf("Press any key\n");
            cgetc(); // conio.h
        }
    #endif /* __CC65__ */
    return 0;
}