Bug#493814: Dangling libgnat.so-4.3 links in adalib directory of gnat, no libgnat.so file
OK, I understand now.
If you want a libgnat.a containing position-independent code, you'd have to
rebuild the gnat-4.3 package for yourself. The changes necessary are quite
small (in gcc/ada/Makefile.in), but I'm unwilling to make them because they do
not correspond to a general case. Not to mention that it would be against
Debian policy to provide a static library compiled with -fPIC.