Enable parallel build gcc on Linux

[Marc Espie]

I've had a fun idea to expose parallel builds errors. 
I'll have a patch for pmake to do it soon, but I assume the idea
is easily applicable to gnu-make as well:

just reverse all dependencies at parsing stage. If Makefile says:
a: b c d

you read:
a: d c b

If the Makefile is correctly specified, it won't change a thing. If it
depends on sequential behavior, hum, oh.... it will be fun.

I could also make it do `random' things, but I don't think it's a good idea
and will gain anything.  As far as recursive makes go, they will normally 
inherit the parent make flags unless the invocation is completely wacky, 
but this could be more systematically asserted, through an env variable
nobody is likely to stumble upon (MAKE_BE_NASTY, anyone ?)