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/t37.c

@@ -0,0 +1,19 @@
+#define BC //**/*
+#define EC */**//
+
+BC
+comment?
+EC
+
+BC comment? EC
+
+#define FOO(x) x
+FOO(abc BC def EC ghi)
+
+#define BAR(x, y) x y
+BAR(abc BC def, ghi EC jkl)
+
+BC
+#define BAZ baz
+EC
+BAZ