Prevent config.h from being clobbered
Fix annoying Perl 5.16 warnings

I.e.

Subroutine Nix::Store::isValidPath redefined at /nix/store/clfzsf6gi7qh5i9c0vks1ifjam47rijn-perl-5.16.2/lib/perl5/5.16.2/XSLoader.pm line 92.

and so on.