As per the Teensyduino 1.48 release thread.

T3.x sketches could compile that had int mai() {} and not void() and setup():

So simply copied over setup.cpp from T3 folder of cores, and then changed setup.c to call main, which simply calls setup and loops calling loop/yield.

Added another endless loop after the call to main() to hang there if the users main returns.

Tested with simple sketch
```
#include <arduino.h>
int main (void) {
  pinMode(13, OUTPUT);
  for(;;) {
    digitalWriteFast(13, HIGH);
    delay(250);
    digitalWriteFast(13, LOW);
    delay(250);
  }
}
```
tried it on T3.6 verified before compile failed and now works.  Also compiled and run a normal app to just make sure it was not totally busted