missing: --run is supported again (as a no-op) for use by older automake

missing: --run is supported again (as a no-op) for use by older automake

Commit v1.12.1-87-ga22717d of 2012-06-21 ("missing: do not touch
timestamps; only warn for out-of-date files") has removed the '--run'
option, since our new preferred calling conventions now imply it; but
if a newer 'missing' is mixed with an already built project that used
an older Automake version (this happened in practice for GNU m4; see:
<http://lists.gnu.org/archive/html/bug-m4/2012-06/msg00001.html>), then
the 'Makefile' in that project will fail due to passing the '--run'
option when trying to bring the project up-to-date.

* lib/missing: Parse and ignore '--run'.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>