Review: Makefile updates

Jeremy Selan <jeremy...@...>


These checkins expose additional build options on the command-line
(needed for custom facility install scripts).
* SOVERSION can now be exposed, and overridden, on the command-line (a
la openimageio)
* The python so can be built with, or without, the "lib" prefix.

This checkin does *not* change any build default values. Default
behavior should be identical.

-- Jeremy

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