where is the main() function located ? can we see its definition in the compiler files like header files?

It was C. No one intends to add backward compatibility for void main. And it was never introduced by microsoft. It was always working like this. In standard C you can make such a program "main(){}", no prototype, by default assumed int, no return. And this is a fully valid C program. So, you can write "void main(){}" there is no conflicts, because this is the way C works. You can just use void instead of int if you don't care about return value. Not recommended, but you can do it. Nothing to do with standards and supports. C just works like this.

See such a program, you even are not obligated to declare the functions before using.
See mixing the return type, and returning nothing in int function.
/*main.c*/
int doX()
void doy(){printf("hello world\n");}
main(){doX();}
/*externaltest.c*/
int doy()
void doX(){doy();}

The program is fully valid under any C compiler.
Moreover. I can add some more hollywar. For example in C you can do like this
void main(int a, int b, int c){ do something }
And it fully works, however the parameters are useless.

As I said, it is just holly war. Is permissible everything the language admits and is not constrained by the language. Everything else, like "is recommended" or "is not recommended" is just a convention. Any thing that should be prohibited, is prohibited (by the language), and you can not use. All the rest is just holly wars.

In my first reply I've told that I agree that it should be avoided, and STOP ON ARGUING ABOUT "should be avoided or not". I've already told, should be. Now I just explain why it works, no matter of compiler, and why it is not as big problem as it is intended to pretend. It is C call to your main function, this is why it works. No matter that C compiler you use. Most people which say that it is bad habit. And they try to scary you with explanation which denotes lack of C knowledge.

Ok, I've just compiled this with GCC 4.4.0
/*x.c*/
void main(int a, void* b, double* c, char d)
{
printf( "hello word from not C\n");
}
G++ detects exactly the void main, and nothing in the parameter list :D. So if you change void to int, everything works. Only have to include stdio.h and return a value.
Anyway, if you write a program which is not portable, it is not a problem while you don't care about portability.