libtool: Support MSYS and MSYS2 file path conversions

libtool: Support MSYS and MSYS2 file path conversions

MSYS does not have cygpath installed by default, so during configure,
a check for cygpath is cached. If cygpath is not found, Window's command
prompt will be used with a forward slash escape on the 'c' flag:
"cmd //c".

Reported: https://savannah.gnu.org/support/?110901

* build-aux/ltmain.in: Choose file path conversion for MSYS(2) by
  whether cygpath is installed.
* m4/libtool.m4: Cache whether cygpath is installed in user path.