Fixed Makefile so it tracks dependencies better

It now tracks main.cpp's dependencies and rebuilds them as needed.

src/preprocesser.hpp

@@ -0,0 +1,62 @@
+/* Copyright (C) 2024 Aryadev Chavali
+
+ * You may distribute and modify this code under the terms of the GPLv2
+ * license.  You should have received a copy of the GPLv2 license with
+ * this file.  If not, please write to: aryadev@aryadevchavali.com.
+
+ * Created: 2024-04-14
+ * Author: Aryadev Chavali
+ * Description: Preprocessor which occurs after lexing before parsing.
+ */
+
+#ifndef PREPROCESSER_HPP
+#define PREPROCESSER_HPP
+
+#include <ostream>
+#include <tuple>
+
+#include "./lexer.hpp"
+
+enum pp_err_type_t
+{
+  OK = 0,
+  EXPECTED_NAME,
+  EXPECTED_STRING,
+  EXPECTED_END,
+  FILE_NONEXISTENT,
+  FILE_PARSE_ERROR,
+  UNKNOWN_NAME,
+};
+
+struct pp_err_t
+{
+  const token_t *reference;
+  pp_err_type_t type;
+  lerr_t lerr;
+
+  pp_err_t();
+  pp_err_t(pp_err_type_t);
+  pp_err_t(pp_err_type_t, const token_t *);
+  pp_err_t(pp_err_type_t, const token_t *, lerr_t);
+};
+
+std::ostream &operator<<(std::ostream &, pp_err_t &);
+
+struct pp_unit_t
+{
+  const token_t *const token;
+  struct
+  {
+    std::string_view name;
+    std::vector<pp_unit_t> elements;
+  } container;
+
+  pp_unit_t(const token_t *const);
+  pp_unit_t(std::string_view, std::vector<pp_unit_t>);
+};
+
+std::vector<pp_unit_t> tokens_to_units(const std::vector<token_t *> &);
+pp_err_t preprocess_use(std::vector<pp_unit_t> &);
+pp_err_t preprocesser(const std::vector<token_t *> &, std::vector<token_t *> &);
+
+#endif