changed C source to use C booleans with backward compatibility

Fix "Under source code control" date for new version.h file.

Sorted the order of symbols printed by "make env".

Test if <stdbool.h> exists and set HAVE_STDBOOL_H accordingly
in have_stdbool.h.  Added HAVE_STDBOOL_H to allow one to force
this value.

Added "bool.h" include file to support use of boolean symbols,
true and false for pre-c99 C compilers.  The "bool.h" include
file defines TRUE as true, FALSE as false, and BOOL as bool:
for backward compatibility.

The sign in a ZVALUE is now of type SIGN, whcih is either
SB32 when CALC2_COMPAT is defined, or a bool.

Replaced in C source, TRUE with true, FALSE with false, and
BOOL with bool.

version.h

@@ -3,8 +3,6 @@
  *
  * Copyright (C) 2023  David I. Bell and Landon Curt Noll
  *
- * Primary author:  David I. Bell
- *
  * 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.
@@ -19,8 +17,8 @@
  * 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:	1990/05/22 11:00:58
- * File existed as early as:	1990
+ * Under source code control:	2023/08/19 17:32:42
+ * File existed as early as:	2023
  *
  * chongo <was here> /\oo/\	http://www.isthe.com/chongo/
  * Share and enjoy!  :-)	http://www.isthe.com/chongo/tech/comp/calc/