In previous versions of SDL the SDL_WINDOWID environment variable would accept a value in decimal or in hex (prefixed by 0x).  However 1.2.10 does not accept values in hexidecimal.  An easy fix is to change the SDL_strtoull function in stdlib/sdl_string.c to understand the 0x prefix.

#ifndef HAVE_STRTOULL
Uint64 SDL_strtoull(const char *string, char **endp, int base)
{
    size_t len;
    Uint64 value;

    // Check for base 16
    if ( !base ) {
        if ( (strlen(string) > 2) && (strncmp(string, "0x", 2) == 0) ) {
            base = 16;
        } else {
            base = 10;
        }
    }        

    len = SDL_ScanUnsignedLongLong(string, base, &value);
    if ( endp ) {
        *endp = (char *)string + len;
    }
    return value;
}