"Remember C is C. No matter what your target board is based on (PC,8051 or PIC)."

Well, yes and no: 'C' is 'C' - but to get anywhere with these sort of processors, you are going to need certain processor-specific to the standard language.

Keil C51 provides extensions such as bit, sbit, sfr, the memory-space keywords data, xdata, etc, a pointer implementation to cope with the different memory spaces, etc, etc, etc...

I'm sure all the other 8051-targetted compilers provide similar extensions.

By careful design, you can isolate these compiler-dependencies; eg,

#if defined( __C51__ )
// For Keil C51, use the m-space keyword extensions
#define  DATA  data
#define IDATA idata
#define PDATA pdata
#define XDATA xdata
#else
// For other compilers, remove the m-space keyword extensions
#define  DATA  
#define IDATA 
#define PDATA 
#define XDATA 
#endif

and similarly for all the other extensions.