make win*.mak more adjustable to the build environment

This patch allows the make and librarian used in the build to be specified on the command line. These need to be forwarded to the zlib build.

I'd rather like to see the win32/64 makefiles nuked and posix.mak being used for all platforms, though. The windows makefile don't allow building win32/win64 and debug/release at the same time and contain a lot of repetitions, probably caused by the underpowered make.