The only 8051-specific bit about it is writing to & reading from SFRs - and translating SFR accesses in assembler to SFR accesses in 'C' is trivial; eg, from the "Simple RS232 Guide" in the FAQs:

SERINIT: ;Initialize serial port
         CLR TR1
         CLR TI
         CLR RI

         MOV SCON,#01011010B ;TI SET INDICATES TRANSMITTER READY.
                             ;MODE 1 REN

         MOV TMOD,#00100001B ;TIMER 1 MODE 8 BIT AUTO RELOAD
         MOV TH1,#0FDH       ;TIMER RELOAD VALUE
         SETB TR1            ;START TIMER

becomes:

void serinit( void )
{
   TR1  = 0;
   TI   = 0;
   RI   = 0;

   SCON = 0x5A;

   TMOD = 0x21;
   TH1  = 0xFD;

   TR1  = 1;
}

I'll leave you to properly comment it, and do something about the "magic numbers"...

See your Keil manuals for specific language extensions for checking & clearing bits...