Restructure endianness library target and gate demo executable in cmakelist file
- Rename
endiannesstarget toendianness-lib - Remove redundant enable_language(C CXX) (already set via project())
- Use CMAKE_CURRENT_SOURCE_DIR/src directly for include dirs and sources instead of intermediate variables
- Drop -Wall -Wextra -Werror compile options from endianness-lib
- Rename demo executable target from endianness_demo to endianness
- Gate demo executable behind new LIB_DEMO_ENABLE option (default OFF)