build: prepare serial over Bluetooth, search for optional BlueZ lib

build: prepare serial over Bluetooth, search for optional BlueZ lib

Adding Bluetooth communication is desirable for all sigrok supported
platforms. The BlueZ library is available on Linux which will receive
support first. Check for the BlueZ library's presence, determine a
HAVE_BLUETOOTH summary state, and extend the HAVE_SERIAL_COMM check.
Print version details for the external library.

This commit extends build support and version information, but does not
yet include the implementation of the serial transport primitives.