improve error reporting

t/porting/cpphdrcheck.t

@@ -216,7 +216,14 @@ sub ok_compile_only($job, $conf, $name) {
     local $Level = $Level + 1;
     my $result = _test_compile_only($job, $conf);
 
-    ok($result, $name);
+    if (ok($result->{ok}, $name)) {
+        note "cmd: $result->{cmd}";
+        note "out: $result->{out}";
+    }
+    else {
+        diag "cmd: $result->{cmd}";
+        diag "out: $result->{out}";
+    }
 }
 
 sub _test_compile_only ($job, $conf) {
@@ -241,18 +248,22 @@ sub _test_compile_only ($job, $conf) {
     }
 
     my $cmd = "$conf->{ccpp} $opts $code 2>&1";
-    note "running '$cmd'";
     my $out = `$cmd`;
 
     chdir $cwd;
 
     unless ($? == 0) {
-        note "'$cmd' failed: $out";
-        return;
+        return
+          +{
+              cmd => $cmd,
+              out => $out,
+          };
     }
 
     return
       +{
+          ok => 1,
+          cmd => $cmd,
           out => $out,
       };
 }
@@ -336,7 +347,13 @@ sub get_source ($keyword) {
 }
 
 # the test code below tries to use at least one language feature
-# specific to that version
+# specific to that version.
+#
+# For now we don't try to do anything real with perl here, but that may change.
+#
+# The perl headers need to be after the C++ headers since the perl headers
+# define many macros that could conflict with the public and non-public
+# like "std::__impl::somenamehere" names that the C++ headers use or define
 __DATA__
 -- cpp11
 #include <iostream>