[gtkada] cosmetic suggestions in Makefiles

[Emmanuel Briot]

Nicolas,

I have looked at your patch and included most of it.

> "./configure --help" claims that it handles CPPFLAGS, but
> src/Makefile.in ignores them.  The patch below should fix that.

There is no C++ in GtkAda, so I am not sure we should be using CPPFLAGS,
which might in fact end up generating errors if there are C++ switches there.
So I did not include that part.

> The distclean target
> - forgets to clean a few generated files
> - forgets to clean a generated  directory
> - cleans gtkada_install_static*.gpr (which are not generated).

I have mostly integrated those, slightly modified. I have added "distclean"
targets to Makefiles that did not have them, for consistency.

> I have written
> "if test -e Makefile; then $(MAKE) distclean; fi" instead of "-$(MAKE) clean"
> and "rm -f" instead of "-rm -f"
> to see unexpected failures.

I did not do that part, since using constructs like "if" adds constraints to the
shell, which is often a pain on Windows for instance. So I have kept the
previous pattern of using "-" at the beginning of the line.

Thanks for the patch

regards
Emmanuel