libcollectdclient: Added header lcc_features.h.

libcollectdclient: Added header lcc_features.h.

This header provides various macros and functions that may be used to
determine the version of libcollectdclient (i.e. collectd). The file will be
auto-created from lcc_features.h.in by configure.

The macro LCC_VERSION (previously defined in client.h) has been moved to this
header as well and has been renamed to LCC_API_VERSION.

This allows for fine grained version checks at compile- and runtime.