Add tradcpp to the repository, and use it as GENCPP (generic preprocessor)

With lots of experimenting, tradcpp (0.4) seems to work way better
than gcc, and it's output is actually valid. We'll go with that for
now.

cde/util/tradcpp/tests/t32.c

@@ -0,0 +1,21 @@
+#define foo(x) "x"
+#define bar(x) 'x'
+#define baz frob
+foo(3)
+bar(3)
+foo(baz)
+bar(baz)
+"baz"
+'baz'
+"foo(baz)"
+"bar(baz)"
+
+#define foo2(x) foo(x)
+#define bar2(x) bar(x)
+foo2(baz)
+bar2(baz)
+
+#define foo3(x) foo2(x)
+#define bar3(x) bar2(x)
+foo3(baz)
+bar3(baz)