Separate dir for the C frontend

[Gioele Barabucci]

Now that the SC has decided to switch from 3.5 to 4.0, I think the time
has come to put the C frontend in its own directory and leave toplevel
gcc/ for shared routines.  I know this move is not simple, but it has to
be done sooner or later, and this major number change is the last
occasion before 5.0 (that will probably come in three or more years).

And please don't postpone this move because of mainline being on stage 3:
the change on version numbering has been announced after the mainline
freeze...

-- 
Gioele Barabucci <barabucc@cs.unibo.it>
) http://cs.unibo.it/~barabucc