m4: Revert commit d6690cc7 to libtool.m4
Reported by Benjamin Gilbert at:
<https://savannah.gnu.org/bugs/?41203>
With recent libtool, this patch did nothing but fixing the output
message of configure.
* m4/libtool.m4: Remove the special handling of NM detection when
cross-compiling mingw32 on cygwin.
