calc 2.14.0.12 release

The following are the changes in this release:

Fixed a number of typos.

Drop support for SunOS, IRIX and MINGW32_NT-5.0 targets.
Drop support for CLOCK_SGI_CYCLE.  Drop testing for __MSDOS__.

Minor improvement of various help files.  Made format of help
files more consistent.

Corrected a few comments in zrandom.c, including a case where
the comment referred to 1007 when it should have used 2^32.

Improved seed() generation.  Improved some comments in seed.c.
Added have_environ.c to build have_environ.h in order to
determine if:

extern char **environ;	/* user environment */

is an valid external symbol.

Fixed documentation that referred to the old additive 55
generator.  We have been using the subtractive 100 in place
of the additive 55 generator for a while now.

Fixed depend rule for custom/Makefile.

Fixed how Makefile variable SHELL is set on macOS vs. Linux.
Using /bin/sh on macOS due to how zsh treats globs by default.

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.

Compiling now tests for _WIN64 as well when testing for _WIN32.

Now assuming that <stdio.h> is available under _WIN32 and _WIN64.

Added some preliminary notes about attempts to compile calc
under Windows 11.

BTW: While we are unable to use Windows 11, we welcome
     Windows 11 developers to try compiling calc natively
     (instead of via a Linux virtual machine).  If you are
     able to compile Windows 11 natively, we would welcome
     GitHub pull requests showing any needed modifications:

	https://github.com/lcn2/calc/pulls

     Please also add notes to the 'Compiling calc under Windows 11'
     section in README.WINDOWS file.

We added Makefile.simple as part of the master branch source
to help those who may be using a make tool that does not support
GNU Make-like features such as "ifeq" and ":=".

Fixed incorrect ustat_dev member ref in seed() builtin that
was part of the unreleased calc v2.12.0.11.