perl: unset NDEBUG after loading Perl headers

Otherwise, loading Perl bindings fails early with[1]

    undefined symbol: Perl_pad_sv at /nix/store/h2jsb5i4yfblr2f3ac2c7zpmlmj7zjym-perl-5.40.0/lib/perl5/5.40.0/XSLoader.pm line 94

Apparently, it's expected behavior by Perl that this symbol only exists
with `DEBUGGING` being set, hence it's used by the headers. However,
`pkgs.perl` from nixpkgs is apparently not built with `-DDEBUGGING`
causing this error.

Now, `NDEBUG` is manually unset after loading the Perl
headers rather than setting `DEBUGGING` causing the error mentioned
above.

I confirmed that this not only fixes the problem described above, but
running the Hydra tests with

    diff --git a/perl/lib/Nix/Store.xs b/perl/lib/Nix/Store.xs
    index dfdd64d28..14788266c 100644
    --- a/perl/lib/Nix/Store.xs
    +++ b/perl/lib/Nix/Store.xs
    @@ -27,6 +27,7 @@ using namespace nix;

     static AsyncIoRoot & aio()
     {
    +assert(false);
	 static thread_local AsyncIoRoot root;
	 return root;
     }

still results in assertion errors.

Finally, added a small install-check that importing `Nix::Store` works
fine.

[1] https://git.lix.systems/lix-project/hydra/issues/69

Change-Id: I58521777eb0f94b766a9813aa4bbd06f9052bd35

perl/default.nix

@@ -57,5 +57,10 @@ perl.pkgs.toPerlModule (
     postUnpack = "sourceRoot=$sourceRoot/perl";
 
     passthru = { inherit perl; };
+
+    doInstallCheck = true;
+    installCheckPhase = ''
+      env PERL5LIB="$PERL5LIB:$out/${perl.libPrefix}" perl -e 'use Nix::Store'
+    '';
   }
 )

perl/lib/Nix/Store.xs

@@ -5,14 +5,12 @@
 // can i make it any more obvious?
 #include "lix/libutil/result.hh"
 
-// Without this, Perl defines NDEBUG, which makes the assertions
-// ineffective and might break Lix
-#define DEBUGGING
-
 #include "EXTERN.h"
 #include "perl.h"
 #include "XSUB.h"
 
+#undef NDEBUG
+
 /* Prevent a clash between some Perl and libstdc++ macros. */
 #undef do_open
 #undef do_close