Add an option to link against static ilmbase and openexr libraries

This adds new cmake variables `ILMBASE_USE_STATIC_LIBS` and `OPENEXR_USE_STATIC_LIBS` which do what they sound like.  The mechanism for preferring static over dynamic libs is taken from `FindBoost.cmake`. 

This also addresses linking order for static libraries:  IlmImf must come before IlmThread in the list of linked libraries or else IlmThread symbols will remain undefined in the resulting libOpenImageIO.so.