Changes to SPI custom build scripts, versioning/disting policy changes

[lgritz]

This mostly concerns builds at SPI, but a tiny bit touches the generic
build scripts. Most of you will want to tune out now, unless you're
especially curious.

Changes for everybody:

• Switching to a 4-part formal version number (e.g. 2.1.3.0), so there
  is an additional (rarely used) CMake variable
  OPENIMAGEIO_VERSION_TWEAK. This gives me a little more flexibility
  with certain tags and releases in the master branch but there are
  very few contexts in which anybody will care about or refer to the
  last tweak number, and in practice we may rarely or never use it.

• New CMake option OIIO_NAMESPACE_INCLUDE_PATCH, if turned on, will
  make the "inner namespace" include the patch number (such as
  namespace OpenImageIO_v2_1_3), whereas the default is the usual
  major/minor (OpenImageIO_v2_1). In general, for external/supported
  releases, we want just major/minor, since within a major/minor
  family we guarantee link compatibility and specifically DON'T want
  to force a recompile for patch updates only. But for studios like
  ours who build for production directly from master, link
  compatibility is not guaranteed at any time, so we make sure even
  down to the patch level is carefully namespaced.

Changes that only have an effect if you're building at SPI:

• The default build (regular old "make") now uses the generic
  default namespace. If you want to make the special namespaced
  version we use for Arnold, build with make OIIO_ARNOLD=1.

• Change SpComp2 versioning from consecutive low integer SpComp2
  versions (like v47, v48) to canonical multidigit numbers
  corresponding to external releases, such as v20101 for OIIO 2.1.1.
  Also install the bins in the same place (even though it's
  nonstandard SpComp2 practice).

• A new build mode enabled by make OIIO_SPIREZ=1 does the kind of
  build we want for an experimental Rez packaging. Some notable details
  and options:

  • REZ_PACKAGE_ROOT=path overrides the usual root area where we
    store the packages.
  • OIIO_REZ_NAME=name overrides the default package name for the
    Rez distribution (default is "OpenImageIO").
  • An appropriate package.py is automatically generated.
  • The Rez variants fold in the SPI compiler compatibility platform,
    Python major/minor version, and Boost version.
  • The build automatically sets the Rpath for the Rez install area
    so that nobody needs to mess with their LD_LIBRARY_PATH to get
    the libraries to be found (especially if you're just importing the
    Python module).

[lgritz]

Would love comments/review, @jware-vc

[jware-vc]

This looks good to me.

[jware-vc]

if we are properly building the rpath into the so's, i think we can comment or remove this line.

[lgritz]

I've updated again. Addressed some of Jon's comments. Also a little cleanup, addition of generating the bit of xml that makes appcfg understand where it lives (we will no longer update the appwrapped command line version as part of the Arnold build), and addition of a README-SPI.md that spells it all out in case somebody needs to do it other than me.