Makefile: Fix in-tree clean/distclean

Makefile: Fix in-tree clean/distclean

Doing 'make clean' or 'make distclean' in a freshly cloned tree results in:

make: *** No rule to make target 'ninja-clean', needed by 'clean'.  Stop.

Make the fallback rules global. While here, change the ninjatool recipe to
always have a zero exit status and thus prevent make to emit a warning.

Fixes: a56650518f5b ("configure: integrate Meson in the build system")
Signed-off-by: Greg Kurz <[email protected]>
Message-Id: <159897001659.442705.15538955005543395950[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>