Not shipping pari.desc in the tarball and other wishlist

[Bill Allombert <allomber@math.u-bordeaux.fr>]

Hello PARI developers,

Some wishlist about our build system:

We should try to remove pari.desc from the tarball. This is
useless since pari.desc can be trivially rebuild with find and cat,
and is not needed for normal build.
(This would save 186kB and reduce the tar.gz by 44kB ).

Secondly, I think we could get rid of src/desc/Makefile at this stage
and move it to Oxx-xx/Makefile.  Handling correctly submakefile is
always difficult.

Third, we should implement a top-level makefile target that take care
of setting up the source tree in a way such that parallel build of 
pari in separate Oxxx-xxx object dir are safe. Currently there is a
small risk of two threads updating the autogenerated .h files
at the same times and producing garbage.

Cheers,
Bill.