All examples, regression tests and tutorials directly looked into
opensubdiv source directory to grab the header files. This is somewhat
convenient during development but they can mistakenly access private
header files.
With this change, when OPENSUBDIV_INCLUDE_DIR is given to cmake,
it will be used as an include search path to build examples etc.
Otherwise it follows the same behavior as before.
Also replaces include references to the files in regression dir
to be relative, and cleanups some copy-paste patterns.
