maybe this could be more readable to use make-style conditions ?

have at the start of the file:

```make
REPORTER ?= JSONMozPerf
```

means that if the user does not set a REPORTER env variable, it will take JSONMozPerf by default. Should we have this reporter by default btw, or use the console reporter by default ?

then, put the FIRST_LOOP_ITERATION stuff between:

```make
ifeq ($(REPORTER), JSONMozPerf)
...
endif
```

Makefile-style conditions are quite cool, because it works like a preprocessor: if they don't match, it's as if the lines are not there at all.

So might be a good idea to make this more readable.