On Windows, find potential libs regardless of file name case.

On Windows, find potential libs regardless of file name case.

* libltdl/m4/libtool.m4 (_LT_CHECK_MAGIC_METHOD),
libltdl/config/ltmain.m4sh (func_mode_link): On Windows,
find potential libs regardless of file name case.
* tests/nocase.at: New test, to check for regressions
of the above.
* Makefile.am: Add above new test.
* doc/libtool.texi (libtool script contents): Document
new variables.
* NEWS: Updated.

Signed-off-by: Peter Rosin <peda@lysator.liu.se>
Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>