Improved source code flow

Changed /*NOTREACHED*/ to not_reached(); and use "attribute.h".

Added .PHONY rule, just after all rule, to Makefiles.

Fixed an improper indentation issue.

attribute.h

@@ -0,0 +1,69 @@
+/*
+ * attribute - control use of attributes in a backward compatible way
+ *
+ * Copyright (C) 2022  Landon Curt Noll
+ *
+ * Calc is open software; you can redistribute it and/or modify it under
+ * the terms of the version 2.1 of the GNU Lesser General Public License
+ * as published by the Free Software Foundation.
+ *
+ * Calc is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+ * or FITNESS FOR A PARTICULAR PURPOSE.	 See the GNU Lesser General
+ * Public License for more details.
+ *
+ * A copy of version 2.1 of the GNU Lesser General Public License is
+ * distributed with calc under the filename COPYING-LGPL.  You should have
+ * received a copy with calc; if not, write to Free Software Foundation, Inc.
+ * 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+ *
+ * Under source code control:	2022/01/21 22:51:25
+ * File existed as early as:	2022
+ *
+ * chongo <was here> /\oo/\	http://www.isthe.com/chongo/
+ * Share and enjoy!  :-)	http://www.isthe.com/chongo/tech/comp/calc/
+ */
+
+
+#if !defined(INCLUDE_ATTRIBUTE_H)
+#define INCLUDE_ATTRIBUTE_H
+
+
+/*
+ * backward compatibility
+ *
+ * Not all compilers support __attribute__ nor do they suuport __has_builtin.
+ * For example, MSVC, TenDRAm and Little C Compiler doesn't support __attribute__.
+ * Early gcc does not support __attribute__.
+ *
+ * Not all compiles have __has_builtin
+ */
+#if !defined(__attribute__) && \
+    (defined(__cplusplus) || !defined(__GNUC__)  || __GNUC__ == 2 && __GNUC_MINOR__ < 8)
+#    define __attribute__(A)
+#endif
+#if !defined __has_builtin
+#    define __has_builtin(x) 0
+#endif
+
+
+/*
+ * not_reached
+ *
+ * In the old days of lint, one could give lint and friends a hint by
+ * placing the token NOTREACHED immediately between opening and closing
+ * comments.  Modern compilers do not honor such commented tokens
+ * and instead rely on features such as __builtin_unreachable
+ * and __attribute__((noreturn)).
+ *
+ * The not_reached will either yield a __builtin_unreachable() feature call,
+ * or it will call abort from stdlib.
+ */
+#if __has_builtin(__builtin_unreachable)
+#    define not_reached() __builtin_unreachable()
+#else
+#    define not_reached() abort()
+#endif /* __has_builtin(__builtin_unreachable) */
+
+
+#endif /* !INCLUDE_ATTRIBUTE_H */