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: a5665051 ("configure: integrate Meson in the build system")
Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <159897001659.442705.15538955005543395950.stgit@bahia.lan>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>