Use $CC -E instead of cpp which can't parse system headers.
https://rt.cpan.org/Public/Bug/Display.html?id=131089

TODO: If CC is specified on the command line (which it is in MacPorts)
use that instead of $Config{cc}.
--- Makefile.PL.orig	2021-11-23 11:09:49.000000000 -0600
+++ Makefile.PL	2021-11-23 11:09:24.000000000 -0600
@@ -3,6 +3,7 @@
 
 use lib '.';
 use inc::Module::Install;
+use Config;
 
 name			'WWW-Curl';
 abstract		'Perl extension interface for libcurl';
@@ -101,9 +102,10 @@
      print "Found curl.h in $curl_h\n";
      my @syms;
      my $has_cpp = 0;
-     open(H_IN, "-|", "cpp", $curl_h) and $has_cpp++;
+     my $cc = $Config{cc};
+     open(H_IN, "-|", $cc, "-E", $curl_h) and $has_cpp++;
      unless ($has_cpp) {
-         warn "No working cpp ($!).  Parsing curl.h in Perl";
+         warn "No working $cc -E ($!).  Parsing curl.h in Perl";
          open(H_IN, "<", $curl_h) or die("Can't open curl.h at path $curl_h, because: ".$!);
      }
      while ( <H_IN> ) {