[CMake] Warning about make 3.81 on cygwin

[Bill Hoffman]

So, with cmake you can use "Unix Makefiles" to build with the microsoft
compiler cl.  This is useful for running -j N parallel builds with visual stuido
2005.   However as of make 3.81 in cygwin this change was made:

http://www.cygwin.com/ml/cygwin-announce/2006-07/msg00008.html


>Note that the --win32 command line option and "MAKE_MODE" environment
>variable are no longer supported in Cygwin's make.  If you need to use a
>Makefile which contains MS-DOS path names, then please use a MinGW
>version of make.
>
>See: <http://mingw.org/>http://mingw.org/ for details on downloading a version of make
>which understands MS-DOS path names.  Please! direct any questions about
>the MinGW version of make to the appropriate MinGW mailing list.

So, cmake will create makefiles that do not work with gmake from cygwin
anymore with "Unix Makefiles".   The problem is the driver letters:

foo: c:/bar

Is now an error in make 3.81, but worked in make 3.80 and less.
So, if you want to use gmake and cl do not use the cygwin version of make,
but rather use the mingw version.  

This only affects the windows cmake, the cygwin cmake will work fine with
make 3.81 for building cygwin based applications.


-Bill