Use POSIX "command -v" construct to test for prerequisite commands
Clarify error messages on failure

This commit introduces the necessary changes necessary
for implementing flow control. None of the socket types
implements the flow control yet. The code will crash when
the flow control is enabled and the thw lwm is reached.

The following commits will add flow-control support for
individual socket types.

C and C++ headers moved from bindings/ to include/, bindings/ removed
--with-c and --with-cpp options to configure removed, C and C++ now built
and installed by default