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.
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