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.

custom/c_sysinfo.c

@@ -136,8 +136,8 @@ STATIC struct infoname sys_info[] = {
      (FULL)E__COUNT},
     {"E__HIGHEST", "highest calc error", NULL,
      (FULL)E__HIGHEST},
-    {"FALSE", "boolean false", NULL,
-     (FULL)FALSE},
+    {"false", "boolean false", NULL,
+     (FULL)false},
     {"FILEPOS_BITS", "file position size in bits", NULL,
      (FULL)FILEPOS_BITS},
     {"FULL_BITS", "bits in a FULL", NULL,
@@ -248,8 +248,8 @@ STATIC struct infoname sys_info[] = {
      (FULL)TOPHALF},
     {"TOPLONG", "top long bit", NULL,
      (FULL)TOPLONG},
-    {"TRUE", "boolean true", NULL,
-     (FULL)TRUE},
+    {"true", "boolean true", NULL,
+     (FULL)true},
     {"USUAL_ELEMENTS", "usual number of elements for objects", NULL,
      (FULL)USUAL_ELEMENTS},
     {"REGNUM_MAX", "highest custom register number", NULL,