The previous answers show how to do it in C and assembler.

I only add a short explanation.
You cannot use setb PCON.7 directly because PCON is not bit addressable. Therefore the SMOD bit CANNOT be defined as a bit address:

SMOD bit ...

and you have to use logical instructions (anl, orl) with the suitable mask to modify the desired bit. Assigning a symbol to the mask, your code will be more legible.

SET_SMOD equ 80h ; or mask
CLR_SMOD equ 7Fh ; and mask

orl PCON,#SET_SMOD ; set SMOD to '1'

anl PCON,#CLR_SMOD ; clear SMOD

Regards,

Alfredo del Rio.