[gtkada] parallel build support

[Emmanuel Briot]

> the value for "PROCESSORS" is set by the $MAKE_JOBS_NUMBER variable, which is always "1" unless $MAKE_JOBS_SAFE is defined.
> 
> However, defining that turns on -j option for gmake.
> 
> So that's probably the issue -- I need to figure out how to defined PROCESSORS without triggering -j, right?


I don't think MAKE_JOBS_NUMBER is related to GtkAda at all. PROCESSORS is a standard
environment variable that we use in src/Makefile.common.in