[build] warn about stray #?endif in gen-cat.nqp

tools/build/gen-cat.nqp

@@ -18,22 +18,30 @@ sub MAIN(*@ARGS) {
         }
         close($fh);
     }
+    my $stderr := nqp::getstderr();
     for @ARGS -> $file {
         say("# From '$file'\n");
         my $fh := open($file, :r);
-        my $in_cond := 0;
-        my $in_omit := 0;
+        my int $in_cond := 0;
+        my int $in_omit := 0;
+        my int $line    := 1;
         while nqp::readlinefh($fh) -> $_ {
             if my $x := $_ ~~ / ^ '#?if' \s+ ('!')? \s* (\w+) \s* $ / {
                 nqp::die("Nested conditionals not supported") if $in_cond;
                 $in_cond := 1;
                 $in_omit := $x[0] && $x[1] eq $backend || !$x[0] && $x[1] ne $backend;
             } elsif $_ ~~ /^ '#?endif' / {
+                unless $in_cond {
+                    nqp::sayfh($stderr,
+                        "#?endif without matching #?if in file $file, line $line"
+                    );
+                }
                 $in_cond := 0;
                 $in_omit := 0;
             } elsif  !$in_omit {
                 print($_) unless $_ ~~ /^ '# ' \w /;
             }
+            $line++;
         }
         close($fh);
     }