gnucap: broken on darwin

Gnucap relies on LD_LIBRARY_PATH both in its build process and in code,
which Darwin does not understand.

pkgs/applications/science/electronics/gnucap/default.nix

···
     changelog = "https://git.savannah.gnu.org/cgit/gnucap.git/plain/NEWS?h=v${version}";
     license = licenses.gpl3Plus;
     platforms = platforms.all;
+    broken = stdenv.isDarwin; # Relies on LD_LIBRARY_PATH
     maintainers = [ maintainers.raboof ];
   };
 }