Build script: Don't use external Pugi for OSL

Now, things are becoming REALLY confusing. The script does build
pugi, but is never telling OIIO to use an external one. Which makes
it to use a bundled one.

Trying to link OSL to a different version of pugi causes a lot of
linking errors.

Interestingly enough, that was me who made OSL to use external pugi
to solve configuration problem. But now i can not reproduce that
anymore.

Ideally we would either link everything against our pugi, or not
compile it at all.

build_files/build_environment/cmake/osl.cmake

@@ -81,11 +81,6 @@ 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