serial: prepare for the absence of libserialport

serial: prepare for the absence of libserialport

Only reference the libserialport header when the library is available.
Allow to always compile the serial.c source file, but optionally end
up with an empty implementation. Make the sr_serial_dev_inst symbol
available outside of HAVE_SERIAL_COMM such that empty stub code can
compile. This prepares the introduction of alternative transports for
serial communication, while all of them remain optional.

The libsigrok serial layer internally uses parity and flow control
symbols which are provided by libserialport. Optionally locally declare
these symbols when libserialport is not available.