libtool upgrade to 2.4.6 - 13/n: add mingw to systems not needing libm.

https://debbugs.gnu.org/cgi/bugreport.cgi?bug=46559

acinclude/libtool.m4

@@ -3845,7 +3845,7 @@ AC_DEFUN([LT_LIB_M],
 [AC_REQUIRE([AC_CANONICAL_HOST])dnl
 LIBM=
 case $host in
-*-*-beos* | *-*-cegcc* | *-*-cygwin* | *-*-haiku* | *-*-pw32* | *-*-darwin*)
+*-*-beos* | *-*-cegcc* | *-*-cygwin* | *-*-haiku* | *-*-mingw* | *-*-pw32* | *-*-darwin*)
   # These system don't have libm, or don't need it
   ;;
 *-ncr-sysv4.3*)