In function maybe_save_numlock_state, file src/wayland/meta-wayland-keyboard.c, META_IS_BACKEND_NATIVE is used without #ifdef checks. This causes build failure when native backend is disabled but wayland is enabled.

  CC       wayland/meta-wayland-keyboard.lo
src/wayland/meta-wayland-keyboard.c:283:35: warning: cast from 'char *' to 'typeof (keyboard)' (aka 'struct _MetaWaylandKeyboard *') increases required alignment from 1 to 8 [-Wcast-align]
src/wayland/meta-wayland-keyboard.c:449:8: warning: implicit declaration of function 'META_IS_BACKEND_NATIVE' is invalid in C99 [-Wimplicit-function-declaration]
  CCLD     libmutter-1.la
copying selected object files to avoid basename conflicts...
ar: `u' modifier ignored since `D' is the default (see `U')
  CCLD     mutter
./.libs/libmutter-1.so: undefined reference to `META_IS_BACKEND_NATIVE'
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Since this function doesn't do any work when 'META_IS_BACKEND_NATIVE (meta_get_backend ())' is false, I think we can make it empty when native backend is disabled.