Revert "Build script: Don't use external Pugi for OSL"

The change did break compilation on default Ubuntu install for Stefan
and on default CentOS install for myself.

This reverts commit 64671e53d2.

build_files/build_environment/cmake/osl.cmake

@@ -81,6 +81,11 @@ if(WIN32)
     ${OSL_EXTRA_ARGS}
     -DPUGIXML_HOME=${LIBDIR}/pugixml
   )
+elseif(UNIX AND NOT APPLE)
+  set(OSL_EXTRA_ARGS
+    ${OSL_EXTRA_ARGS}
+    -DPUGIXML_HOME=${LIBDIR}/pugixml
+  )
 elseif(APPLE)
   # Make symbol hiding consistent with OIIO which defaults to OFF,
   # avoids linker warnings on macOS