Peter M. Groen / libmodbus

- 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

- partial revert of 93ce13656103cbb459ebd630d055a008dbb059f3
- it was a bit confusing to always edit modbus.h.in
- the header inclusion still unchanged (<modbus.h>)

The library is now simpler to include (only one <modbus.h>)
To avoid confusion MB_VERSION defines have been renamed to
LIBMODBUS_VERSION.

Inspired by the Clutter project and work done by Florian Forster.