From ArchWiki

Nice work but there is a problem with the current gcc version.

I tried several gnat compilers from libre.adacore.com and gnuada.sourceforge.net and actually, only one works. The others fails to compile with the option --mtune=generic and thus the configure script of gcc stops quickly. The only one working is gnat-gcc-4.2.0-r7.suse_9_2.i686.rpm, maybe because this is the only 4.2 binary version ? However it doesn't work yet immediately because it fails to find libiconv.so.2. It so needed to download gnat-tools-libiconv-1.11-r7.suse_9_2.i686.rpm and then to add ~/ADA_BUILD/RPM/opt/gnat/tools/lib to the LD_LIBRARY_PATH environment variable. Then gcc works and it can be used to compile gnat as you describe it well.