or you could just re-write the routine to

void write_adc_byte(unsigned char output_data) {
unsigned char index;

for(index = 0; index < 8; index++) // Send 8 bits to the adc
{
ADC_DIN = ((output_data & 0x80) ? 1 : 0);
output_data=output_data << 1; // Shift the byte by one bit
ADC_CLK = HIGH; // Clock the data into the adc
adc_delay();
ADC_CLK = LOW;
}


adc_delay();
}