Make deps: Fix detection/linking of PugiXML on Linux

This time both full `make deps` and final compilation is tested on
a freshly installed CentOS 7.

The thing is: OpenImageIO is not configured to use an external PugiXML
library, so it was compiling its own.
At the same time the OpenShadingLanguage library was commanded to use
an externally compiled PugiXML. This caused some sort of discrepancy
which lead to Blender-link-time errors. Could be linking error, could
be namespace related, could be ABI related. In any case since we do
have PugiXML in the OpenImageIO already lets just stick to it.

build_files/build_environment/CMakeLists.txt

@@ -91,7 +91,11 @@ include(cmake/openvdb.cmake)
 include(cmake/python.cmake)
 include(cmake/python_site_packages.cmake)
 include(cmake/numpy.cmake)
-include(cmake/pugixml.cmake)
+if(UNIX AND NOT APPLE)
+  # Rely on PugiXML compiled with OpenImageIO
+else()
+  include(cmake/pugixml.cmake)
+endif()
 
 if(WITH_WEBP)
   include(cmake/webp.cmake)

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
@@ -116,9 +111,17 @@ add_dependencies(
   external_zlib
   external_flexbison
   external_openimageio
-  external_pugixml
 )
 
+if(UNIX AND NOT APPLE)
+  # Rely on PugiXML compiled with OpenImageIO
+else()
+  add_dependencies(
+    external_osl
+    external_pugixml
+  )
+endif()
+
 if(WIN32)
   if(BUILD_MODE STREQUAL Release)
     ExternalProject_Add_Step(external_osl after_install