Add depmode=msvcmsys for Microsoft Visual C++ on MSYS.

Add depmode=msvcmsys for Microsoft Visual C++ on MSYS.

* lib/depcomp [msvisualcpp]: Fork fewer processes. Filter out
libtool in the preprocessor invocation (as is done in
depmode=cpp). Silence compiler stderr.
[msvcmsys]: New depmode as a derivative of depmode=msvisualcpp.
msvcmsys transforms any backslashes into forward slashes to
make the grep in depend.m4 match, instead of the "cygpath -u"
that is used in msvisualcpp.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>