Small changes in Ada handling in Gentoo

[george@gentoo.org]

Dear Ada users.

I have finally implemented the multiple gnat handling in full,
resolving bug
#151343. The operational procedures on user side remain largely the
same ,
however there is one modification everybody using Ada
compilers in Gentoo should be aware of.

Now (as was discussed in #151343) gnatbuild.eclass, gnat.eclass and
eselect-gnat  support the notion of "primary" compilers. The idea here
is
that most users will want one or, possibly, two variants of Ada
compilers
used for everyday work at most, while, occasionally, wishing to test
some
other variant(s).

These "everyday" compilers (designated "primary" further on) have to
be listed
in /etc/ada/primary-compilers. The file simply contains a listing of
gnat
profiles user desires to designate primary, one per line. Mine, for
example,
contains:

gerr@dornoir ~ $ cat /etc/ada/primary_compilers
x86_64-pc-linux-gnu-gnat-gcc-4.2
x86_64-pc-linux-gnu-gnat-gpl-4.1

All Ada libs will be built only for these compilers and not for any
other
installed. However "eselect gnat set" will allow user to set
any installed gnat, as before. Thus we avoid ABI issues when updating
packages that have some Ada lib as a dependency (there are already a
few) via
enforcing a consistent set of ABIs, cut compilation time (libs are now
not
getting built for an "occasional" compiler) while still providing an
ability
to use any gnat in portage.

As a safeguard gnatbuild.eclass has also been modified to
populate /etc/ada/primary-compilers, if this file does not exist yet,
with
the corresponding profile upon emerging gnat for the first time.
Please,
however, take your destiny in your own hands and update that file
accordingly
to your desires.

George