t/porting/cpphdrcheck.t: improve error reporting

and make not finding a C++ compiler fail for now, to make it easier
to see the cases where we don't find one but expected to find one.

t/porting/cpphdrcheck.t

@@ -140,7 +140,16 @@ CODE
 
     note "test run for $ccpp";
     my $out = test_run({ ccpp => $ccpp, code => \$ccpp_test_code }, $cfg);
-    unless ($out && $out->{stdout} && $out->{stdout} eq "OK\n") {
+    unless ($out && $out->{run_stdout} && $out->{run_stdout} eq "OK\n") {
+        diag "build: $out->{build_cmd}";
+        diag "build output: $out->{build_out}";
+        diag "build exit: $out->{build_exit}";
+        diag "run cmd: $out->{run_cmd}" if $out->{run_cmd};
+        diag "run stdout: $out->{run_stdout}" if $out->{run_stdout};
+        diag "run stderr: $out->{run_stderr}" if $out->{run_stderr};
+        diag "run exit: $out->{run_exit}" if defined $out->{run_exit};
+        # for now fail if no C++ compiler
+        die "No C++ compiler found";
         return;
     }
 
@@ -176,7 +185,7 @@ CODE
         note "probe $ccpp for standard C++$std with $arg";
         my $code = get_source("cpp$std");
         my $out = test_run({ ccpp => $ccpp, code => \$code, opts => $arg }, $cfg);
-        if ($out && $out->{stdout} && $out->{stdout} eq "OK\n") {
+        if ($out && $out->{run_stdout} && $out->{run_stdout} eq "OK\n") {
             note "found $std";
             $stds{$std} = $arg;
         }
@@ -186,31 +195,6 @@ CODE
     return $cfg;
 }
 
-# perform a test run to see if a compiler works
-# $conf can be empty to unix-like defaults, see test_build() for more
-sub test_run ($job, $conf) {
-    my $dir = File::Temp->newdir();
-    chdir "$dir"
-      or die "Cannot chdir to temp directory '$dir': $!";
-    my $build = eval { _test_build($job, $conf); }
-      or note "test_build: $@";
-    my $result;
-    if ($build) {
-        my $exit = system "$build->{exe} >stdout.txt 2>stderr.txt";
-        $result =
-          +{
-              %$build,
-              exit => $exit,
-              stdout => scalar _slurp("stdout.txt"),
-              stderr => scalar _slurp("stderr.txt"),
-          };
-    }
-    chdir $cwd
-      or die "Cannot chdir back to '$cwd': $!";
-
-    $result;
-}
-
 sub ok_compile_only($job, $conf, $name) {
     our $Level;
     local $Level = $Level + 1;
@@ -268,9 +252,29 @@ sub _test_compile_only ($job, $conf) {
       };
 }
 
+# perform a test run to see if a compiler works
+# $conf can be empty to unix-like defaults, see test_build() for more
+sub test_run ($job, $conf) {
+    my $dir = File::Temp->newdir();
+    chdir "$dir"
+      or die "Cannot chdir to temp directory '$dir': $!";
+    my $result = _test_build($job, $conf);
+    if ($result->{exe}) {
+        my $cmd = "$result->{exe} >stdout.txt 2>stderr.txt";
+        my $exit = system $cmd;
+        $result->{run_exit} = $exit;
+        $result->{run_cmd} = $cmd;
+        $result->{run_stdout} = scalar _slurp("stdout.txt");
+        $result->{run_stderr} = scalar _slurp("stderr.txt");
+    }
+    chdir $cwd
+      or die "Cannot chdir back to '$cwd': $!";
+
+    $result;
+}
+
 # build the supplied code to test we can invoke the compiler
 # and so the caller can run it
-
 sub _test_build ($job, $conf) {
     $conf ||= { type => "unix" };
 
@@ -297,21 +301,25 @@ sub _test_build ($job, $conf) {
 
     my $cmd = "$job->{ccpp} $opts $code 2>&1";
     note "running '$cmd'";
-    my $out = `$cmd`;
+    my $result =
+      +{
+        build_cmd => "$cmd\n",
+       };
+    my $out = `$cmd` // "";
+    $result->{build_out} = $out;
+    $result->{build_exit} = $?;
     unless ($? == 0) {
         note "'$cmd' failed: $out";
-        return;
+        return $result;
     }
 
     my $exe = "a.out$_exe";
     unless ($^O eq "MSWin32") {
         $exe = "./$exe";
     }
-    return
-      +{
-          exe => $exe,
-          out => $out,
-      };
+    $result->{exe} = $exe;
+
+    return $result;
 }
 
 sub _slurp ($filename) {