I wanted some pkgconfig (http://en.wikipedia.org/wiki/Pkg-config) support to make it easier to build against OCIO.

Part of this needed burning the ocio version into the .pc file, so I rejigged things a little so there is only one place to
change the OCIO_VERSION

Notes:
* added some basic pkg-config .pc building
* moved OCIO_VERSION into top most CMakeLists.txt
* added OpenColorVersion.h.in which gets configured() at build time with OCIO_VERSION
* added ${CMAKE_BINARY_DIR}/export/ to a few CMakeLists.h so they can find the resloved OpenColorVersion.h
* added OpenColorIO.pc.in which gets configured() at build time with OCIO_VERSION and CMAKE_INSTALL_PREFIX

.malcolm