serial: introduce more general "have serial comm" feature flag

Introduce the HAVE_SERIAL_COMM identifier, which gets derived from, but
need not be identical to the HAVE_LIBSERIALPORT condition.

Derive the NEED_SERIAL automake condition from the general availability
of serial communication not the specific libserialport library.

Adjust source code references. Stick with HAVE_LIBSERIALPORT where the
specific library is meant, but switch to HAVE_SERIAL_COMM where the
availability of serial communication in general is meant.