Enabling native compilation by default when libgccjit is present

[Andrea Corallo]

Hi all,

ATM native compilation is off by default and must be enabled manually
when configuring.

IIRC was mentioned by Eli (forgive me if I'm mistaken) that we might
want to enable it automatically when configure identifies the presence
of libgccjit on the system.

That's just a reminder cause *if* we want to do this I guess this is the
last call for 28.

Regards

  Andrea