Peter M. Groen / libmodbus

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

The response timeout of the client must be higher than the server
to handle sleep/flush operations done on server side.

This change has introduced by 83c34102.
Fix unit tests on some systems.

This fixes the message
"error: 'for' loop initial declarations are only allowed in C99 mode",
spotted during cross-compiling of libmodbus.

The compiler (gcc) actually does support -std=c99 but it gets not
enabled by default. So a solution would be to enforce c99 mode
via CFLAGS, but this will knock out all compilers which do not
support this mode.
Moving the declaration out of the loop initialisation, seems to be
the simpler solution.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>

Function modbus_receive_from has no implementation in the library and is not used anywhere.

It concerns only the files in 'tests' directory.

Protects against crafted write requests with a large quantity but a small
byte count. If address + quantity was in the mapping space of the server
and quantity greater than the response size, it was possible to crash
the server.

The sleep/flush sequence improves the handling of following requests.

Thank you MarjanTomas and SwissKnife.

Thank you Petr Gladkiy and MarjanTomas.
Sorry for the delay...

From Michael Heimpold:
Just a nitpick: the INADDR_* macros are defined in host byte order,
compare INADDR_LOOPBACK. So it would be reasonable to wrap this with
htonl(INADDR_ANY). However, every experienced socket programmer should
know that INADDR_ANY equals 0.0.0.0, passing it through htonl does not
have any effect.

- protect against NULL IP address BTW
- update documentation

- allow an empty string or NULL for node argument
- protect against NULL in service argument
- new test for NULL service
- update documentation

Thanks to vgrin for the report

It's never too late to remove 261 lines of code ;)

Thanks to Marjan Tomas

Thanks to Marjan Tomas