Question

My software has one main for normal use and a different one for unit tests.
I would just love it if there was an option to gcc to specify which “main” function to use.

Answer 1

Put them in separate files, and specify one .c file for normal use, and one .c file for testing.

Alternately, #define testing on the commandline using test builds and use something like:

int main(int argc, char *argv[])
{
#ifdef TESTING
    return TestMain(argc, argv);
#else
    return NormalMain(argc, argv);
#endif
}

int TestMain(int argc, char *argv[])
{
    // Do testing in here
}

int NormalMain(int argc, char *argv[])
{
    //Do normal stuff in here
}