Improve seed()

Added charbit.h to define CALC_CHARBIT which is either CHAR_BIT
from <limits.h> or 8 or a value set by the Makefile variable
CALC_CHARBIT.  Added have_limits.h to determine if <limits.h>
is a system include file.

charbit.c

@@ -0,0 +1,67 @@
+/*
+ * charbit - determine what CHAR_BIT is and define CALC_CHARBIT
+ *
+ * Copyright (C) 2021  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:	2021/12/07 20:57:50
+ * File existed as early as:	2021
+ *
+ * chongo <was here> /\oo/\	http://www.isthe.com/chongo/
+ * Share and enjoy!  :-)	http://www.isthe.com/chongo/tech/comp/calc/
+ */
+
+/*
+ * usage:
+ *	charbit
+ *
+ * This prog outputs several defines:
+ *
+ *	CALC_CHARBIT
+ *		after including have_limits.h and perhaps <limits.h>,
+ *		output CALC_CHARBIT as CHAR_BIT (from <limits.h>, or as 8.
+ */
+
+#include <stdio.h>
+#include "have_limits.h"
+#if defined(HAVE_LIMITS_H)
+#include <limits.h>
+#endif
+
+
+#include "banned.h"	/* include after system header <> includes */
+
+
+int
+main(void)
+{
+	printf("#include \"have_limits.h\"\n");
+	printf("#if defined(HAVE_LIMITS_H)\n");
+	printf("#include <limits.h>\n");
+	printf("#endif\n\n");
+#if defined(CHAR_BIT)
+
+	printf("#define CALC_CHARBIT (CHAR_BIT) /* from <limits.h> */\n");
+
+#else /* CHAR_BIT */
+
+	printf("#define CALC_CHARBIT (8) /* no CHAR_BIT, assume 8 */\n");
+
+#endif /* CHAR_BIT */
+
+	/* exit(0); */
+	return 0;
+}