Default MAKEFLAGS to -j

[tenderlove]

This tells make to run in parallel by default. If I do

$ rake clobber; time rake compile

On the main branch it's like this:

________________________________________________________
Executed in    9.71 secs    fish           external
   usr time    8.41 secs  128.00 micros    8.41 secs
   sys time    1.16 secs  530.00 micros    1.16 secs

On this branch it's like this:

________________________________________________________
Executed in    3.89 secs    fish           external
   usr time   11.49 secs  127.00 micros   11.49 secs
   sys time    2.00 secs  553.00 micros    2.00 secs

So this buys me about 6 seconds on my machine