[build] parallel build for firmwares

on OSX using sysctl seems to be faster than using system_profile and also more robust
(output is not named differently)
[build] only build firmware in parallel with J=AUTO

by passing J=AUTO from toplevel make it will auto-detect the number of CPUs
Number of processes can also be explicitly set with e.g. J=4
Calling make without a J variable is the same as J=1 (serial compilation)
[build] make sure gcc version is printed at the beginning
Merge pull request #683 from paparazzi/multiproc_compilation

[build] parallel build for firmwares

- by passing J=AUTO from toplevel make it will auto-detect the number of CPUs
- umber of processes can also be explicitly set with e.g. J=4
- calling make without a J variable is the same as J=1 (serial compilation)

e.g. ```make AIRCRAFT=Quad_LisaM_2 ap.compile J=AUTO```