CMake: quiet references to undeclared variable warnings

These warnings can reveal errors in logic, so quiet them by checking
if the features are enabled before using variables or by assigning
empty strings in some cases.

- Check CMAKE_THREAD_LIBS_INIT is set before use as CMake docs
  note that this may be left unset if it's not needed.
- Remove BOOST/OPENVDB/VULKAN references when disable.
- Define INC_SYS even when empty.
- Remove PNG_INC from freetype (not defined anywhere).

source/blender/nodes/function/CMakeLists.txt

@@ -16,6 +16,8 @@ set(INC
   ../../../../intern/guardedalloc
 )
 
+set(INC_SYS
+)
 
 set(SRC
   nodes/node_fn_align_euler_to_vector.cc