Rename averaging target to averaging-lib and make demo executable optional
- Remove redundant enable_language(C CXX) call
- Fix INSTALL_INTERFACE include dir (was pointing to a generic "include" path instead of the actual source include dir)
- Remove -Wall -Wextra -Werror from the library's compile options
- Gate the demo executable behind a new LIB_DEMO_ENABLE option (default OFF), renamed from averaging_demo to averaging