libtool: break all the way out when a good nm is found

libtool: break all the way out when a good nm is found

The current code tries to locate a compatible nm tool.  It starts with
a prefixed nm tool (great!) and includes a plain nm too (that's fine).
The problem is that the code searches for the prefixed nm before the
plain nm (normally fine), but doesn't break once it has found a valid
match, and the plain nm ends up the winner.

Report and analysis by Mike Frysinger.

* m4/libtool.m4 (LT_PATH_NM): Break all the way out on a good match.

Signed-off-by: Peter Rosin <peda@lysator.liu.se>