Cleanup: CMake: Modernize bf_intern_openvdb dependencies

This follows the other CMake "modernization" commits, this time for
`bf_intern_openvdb` and the OpenVDB dependency itself.

The difference with this one is that `intern/openvdb` becomes an
"optional" dependency itself. This is because downstream consumers often
want to include this dependency rather than openvdb directly, so this
target must also be optional. Optional, in this case, means the target
always exists but may be entirely empty.

Summary
- If you are using BKE APIs to access openvdb features, then use the
  `bf::blenkernel` target
- If you are only using `intern/openvdb` APIs then use the
  `bf::intern::optional::openvdb` target (rare)
- For all other cases, use the `bf::dependencies::optional::openvdb`
  target (rare)

context: https://devtalk.blender.org/t/cmake-cleanup/30260
Pull Request: https://projects.blender.org/blender/blender/pulls/137071

intern/openvdb/CMakeLists.txt

@@ -2,36 +2,25 @@
 #
 # SPDX-License-Identifier: GPL-2.0-or-later
 
-set(INC
-  .
-)
-
-set(INC_SYS
-)
-
-set(SRC
-  openvdb_capi.h
-  openvdb_fwd.hh
-)
-
-set(LIB
-  PRIVATE bf::intern::guardedalloc
-)
-
 if(WITH_OPENVDB)
-  add_definitions(-DWITH_OPENVDB ${OPENVDB_DEFINITIONS})
-
-  list(APPEND INC_SYS
-    ${OPENVDB_INCLUDE_DIRS}
+  list(APPEND INC
+    PUBLIC .
   )
 
   list(APPEND SRC
+    openvdb_capi.h
+    openvdb_fwd.hh
     openvdb_capi.cc
   )
 
   list(APPEND LIB
-    ${OPENVDB_LIBRARIES}
+    PUBLIC bf::dependencies::optional::openvdb
   )
+
+  blender_add_lib(bf_intern_openvdb "${SRC}" "${INC}" "${INC_SYS}" "${LIB}")
+else()
+  # Empty target
+  add_library(bf_intern_openvdb INTERFACE)
 endif()
 
-blender_add_lib(bf_intern_openvdb "${SRC}" "${INC}" "${INC_SYS}" "${LIB}")
+add_library(bf::intern::optional::openvdb ALIAS bf_intern_openvdb)