This commit expands the Windows-specific code in rBdca9aa0053f7 and Linux-specific code in rB33b7d53df08a.
It also fixes a capitalization issue in FindPugiXML.cmake
Reduce the number of possible locations used to find libraries,
to simplify troubleshooting.
Only keep '*_ROOT_DIR' and the path used by 'install_deps.sh'.
Basic idea is to check whether OIIO is compiled with embedded PugiXML parser
and if so use PugiXML from OIIO, otherwise find a standalone PugiXML library.