[arch-general] MAKEFLAGS remodelation

[Kalrish Bäakjen]

My goal was to make it generic. SCons and Waf support the "-j" option, but
that is coincidental. MAKEFLAGS is for make; why tweak it to make it work
with other build systems? And, if other build systems support some options
(for debugging, for output supression, for whatever), we need a way to tell
them.

I'm not sure and I can't test it, but I think that your example would be
broken if MAKEFLAGS was "-j N" instead of "-jN".

Anyway, this was just about names. In practice, and as you point out, you
can safely assume that every modern build system accepts "-j".

Thanks,
Kalrish