$ gcc -o scriptedmain scriptedmain.c scriptedmain.h
$ ./scriptedmain
Main: The meaning of life is 42
$ gcc -o test test.c scriptedmain.c scriptedmain.h
$ ./test
Test: The meaning of life is 42

int meaning_of_life();

#include <stdio.h>

int meaning_of_life() {
    return 42;
}

int __attribute__((weak)) main() {
    printf("Main: The meaning of life is %dn", meaning_of_life());

    return 0;
}

#include "scriptedmain.h"
#include <stdio.h>

extern int meaning_of_life();

int main() {
    printf("Test: The meaning of life is %dn", meaning_of_life());
    return 0;
}

C:>gcc -o scriptedmain scriptedmain.c scriptedmain.h
c:/strawberry/c/bin/../lib/gcc/i686-w64-mingw32/4.4.3/../../../../i686-w64-mingw32/lib/libmingw32.a(lib32_libmingw32_a-crt0_c.o): In function `main':
/opt/W64_156151-src.32/build-crt/../mingw-w64-crt/crt/crt0_c.c:18: undefined reference to `WinMain@16'
collect2: ld returned 1 exit status

$ gcc -o scriptedmain -mwindows scriptedmain.c scriptedmain.h
c:/mingw/bin/../lib/gcc/mingw32/4.5.2/../../../libmingw32.a(main.o):main.c:(.text+0x104): undefined reference to `WinMain@16'
collect2: ld returned 1 exit status

#include <stdio.h>

int meaning_of_life() {
    return 42;
}

#ifdef SCRIPTEDMAIN

int main() {
    printf("Main: The meaning of life is %dn", meaning_of_life());

    return 0;
}

#endif

gcc -o scriptedmain -DSCRIPTEDMAIN scriptedmain.c scriptedmain.h