gnat: Attempt to fix random build failures

Sometimes the build failes with:

  In file included from ../../gcc-4.4.6/gcc/ada/seh_init.c:44:
  ../../gcc-4.4.6/gcc/system.h:418: error: conflicting types for 'strsignal'
  /nix/store/6h129q168ahnl2nzw6azr239cba884ng-glibc-2.18/include/string.h:560: note: previous declaration of 'strsignal' was here

and sometimes it doesn't.  Hopefully disabling parallel builds fixes
this.

http://hydra.nixos.org/build/7179481