warning: cast discards qualifiers from pointer target type

char *stristr(const char *string,const char *substring)
{
size_t stringlength = strlen(string);
char *stringlowered = malloc(stringlength + 1);
strcpy(stringlowered,string);
tolower2(stringlowered); // in my source it has a different name,sorry.

char *substringlowered = malloc(strlen(substring) + 1);
strcpy(substringlowered,substring);
tolower2(substringlowered); // in my source it has a different name,sorry.

const char *returnvalue = strstr(stringlowered,substringlowered);
if(returnvalue != NULL)
{
    size_t returnvaluelength = strlen(returnvalue);
    returnvalue = string;
    returnvalue += stringlength - returnvaluelength;
}

free(stringlowered);
free(substringlowered);

return (char *)returnvalue;
}

编辑：
在glibc 2.15的strstr()源代码中：

return (char *) haystack_start; // cast to (char *) from const char *