Use the "modern" cmake way of naming/passing along targets; as a bonus,
this lets us bump the version of openexr we ship as a submodule to be
the most recent release.
Fixes#478, which was due to pbrt not handling having a modern version of
openexr installed locally.
Normalize unicode strings for user-supplied names (objects, materials,
media, etc.) Note that there is no need to normalize strings for things
like the name of the selected sampler, light source types, or the
parameters provided to pbrt objects, as all of the valid ones are plain old
ASCII text. We also intentionally do not normalize pathnames, as doing so
can cause all sorts of trouble.
