Use positive source filtering for the Perl bindings

flake.nix

@@ -477,7 +477,15 @@
             passthru.perl-bindings = with final; perl.pkgs.toPerlModule (currentStdenv.mkDerivation {
               name = "nix-perl-${version}";
 
-              src = self;
+              src = fileset.toSource {
+                root = ./.;
+                fileset = fileset.intersect baseFiles (fileset.unions [
+                  ./perl
+                  ./.version
+                  ./m4
+                  ./mk
+                ]);
+              };
 
               nativeBuildInputs =
                 [ buildPackages.autoconf-archive

perl/Makefile

@@ -1,6 +1,12 @@
 makefiles = local.mk
 
-GLOBAL_CXXFLAGS += -g -Wall -std=c++2a -I ../src
+GLOBAL_CXXFLAGS += -g -Wall -std=c++2a
+
+# A convenience for concurrent development of Nix and its Perl bindings.
+# Not needed in a standalone build of the Perl bindings.
+ifneq ("$(wildcard ../src)", "")
+  GLOBAL_CXXFLAGS += -I ../src
+endif
 
 -include Makefile.config