[Ada] Reenable parallel build

[Eric Botcazou]

> The old commit did not fully document the reasons indeed, but I did
> actually recently investigate where this comment came from, and it is
> actually still relevant: the gnattools target is not safe wrt parallel
> builds (and there may be also an issue if gnatlib and gnattools are
> run in parallel), so your patch is not OK, unfortunately.

Can't we distinguish gnatlib from gnattools though and move the .NOTPARALLEL 
to gnattools/Makefile.in?

-- 
Eric Botcazou