Peter M. Groen / libmodbus

https://github.com/stephane/libmodbus/issues/248

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>

They are already included in these Linux distributions.

- only check if documentation tools are installed in acinclude.m4
- don't provide built documentation in dist tarball
- simpler Makefile
- display warning message and check result

Thank you MarjanTomas and SwissKnife.

- depends of automake 1.11
- C99
- pic-only
- more warning C flags
- remove obscure checks
- don't print directories
- add libmodbus.pc to clean files
- remove -Werror

- avoid export of _modbus_rtu_set_rts and change its name to avoid
  confusion
- namespace RTU functions and constants
- use_rts is renamed rts
- add missing '_' in function name
- remove useless debug message in usual write
- wrong message about 'not supported'
- add check for TIOCM_RTS support
- block RTS flow control when not available

Thank you Ivo De Decker.

All related scripts and config files have been updated too.

The two modes are complementary, MODBUS_ERROR_RECOVERY_LINK handles
errors at data link level (bad file descriptor, timeout, etc) and
MODBUS_ERROR_RECOVERY_PROTOCOL checks Modbus error (eg. invalid
function code or trame length).

This change introduces the use of the Sleep function for Windows.
Some duplicated code has been moved from backends to modbus core.
A new debug message is now available when a flush occurs.

The unit tests are now based on this error recovery code.

Each function is decribed in one txt file. The file libmodbus.txt
offers an overview of the library.

It's possible to generate HTML and man outputs from the txt file.
The documentation can be generated only if you have the required
tools (asciidoc and xmlto).

Better results are obtained with AsciiDoc v8.6+

- removed ENOTCONN errno not supported
- set WINVER to Windows XP (and above) and add ws2_32.dll. So older
  Windows versions aren't supported.

Extracted from octo/libmodbus ff/ipv6 a6ae017c82e

Patch provided by Barry Brumbine.

- removed comment not required anymore with -Wall -Werror