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.

cal/ellip.cal

@@ -1,7 +1,7 @@
 /*
  * ellip - attempt to factor numbers using elliptic functions
  *
- * Copyright (C) 1999  David I. Bell
+ * Copyright (C) 1999,2023  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
@@ -53,7 +53,7 @@
  *	B is the limit of the primes that make up the high power that the
  *	point is raised to for each factoring attempt (default 100).
  *	force is a flag to attempt to factor numbers even if they are
- *	thought to already be prime (default FALSE).
+ *	thought to already be prime (default false).
  *
  * Making B larger makes the power the point being raised to contain more
  * prime factors, thus increasing the chance that the order of the point