Makefile: add comment on CYGWIN_V15_WIN32API

Makefile: add comment on CYGWIN_V15_WIN32API

There is no documented, reliable, and future-proof method to
determine the installed w32api version on Cygwin. There are many
things that can be done that will work frequently, except when they
won't.

The only sane thing is to follow the guidance of the Cygwin
developers: the only supported configuration is that which the
current setup.exe produces, and in the case of problems, if the
installation is not up to date then updating is the first required
action.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>