I know that there is a solution to it, to #include the body of function to both copy, but this is not quite a typical use of #include in C, is it, potentially causing confusion.

if atypical is causing confusions, you better replace the PC types you work with, with embedded guys.

anyhow, how confusing can something like the below be?

you write one .c file xxx.c

in xxx.c you do the following:
#ifdef INT
void IntFunc (void)
#else
void Mainfunc (void)
#endif
{


Erik