I have written a Code to search first occurrence of a substring str2 in a given string str1 But i am getting [-Wdeprecated-declarations] on main.c:9:2

Question

#include<stdio.h>
#include<string.h>

int main()
{
    char str[100], ch;
    int i, Flag;

    Flag = 0;

    printf("\n Please Enter any String : ");
    gets(str);

    printf("\n Please Enter the Character that you want to Search for :");
    scanf("%c", &ch);

    for(i = 0; i <= strlen(str); i++)
    {
        if(str[i] == ch)
        {
            Flag++;
            break;
        }
    }
    if(Flag == 0)
    {
        printf("\n Sorry!! We haven't found the Search Character '%c'", ch);
    }
    else
    {
        printf("\n The First Occurrence of the Search Element '%c' isat Position %d ", ch, i + 1);
    }

    return 0;
}