This is the system file you should include as a matter of last resort if you are developing for the ATMega8 but your actual board is not listed in the sys folder. ie it is for a generic ATMega8 board. This may also be because you have created your own ATMega8 based board. Consequently the library assumes that all processor pins have header pins.
It will make sure that your compiler settings sre appropriate: ie ATMega8
NB Support for the ATMega8 may be deprecated in future releases since it has little memory and little ability compared to the pin-compatible ATMega168 for just a 'few cents more'. The main issue is the 8k progrsm space. As our projects become more complex then we have to decide whether or not to use floating point maths (eg real numbers). This 'luxury' requires the floating point library which immediately adds a 'hit' of about 2.5kb of program space which for the ATMega8 is over a quarter of its total size. So here is the problem: do we continue to support 8k processors or do we make 'real numbers' be built in? Let me know what you think... My own thoughts are 'libraries should help the developer of complex applications' and so the ATMega8 should go the same way as 'Windows for Workgroups', 'MS/DOS' - it has seen its day but the future is different.