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).
66dee44088