Make support for parallel scanners a configure time option

Found on the %1.0.26 milestone. Suggested to add an --enable-parallel option to the configure script.

In view of the existing --with-usb option, --with-parallel might be a better choice. On the other hand, the enable/disable options are grouped as "Optional Features" in the ./configure --help output whereas with/without options list under "Optional Packages".

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