Add experimental cmake-based build system for Windows.

include/apr.hwc is almost exactly the same as apr.hw; it uses
variables for cmake-time feature selection so that cmake
can easily generate apr.h.


git-svn-id: https://svn.apache.org/repos/asf/apr/apr/trunk@1516542 13f79535-47bb-0310-9956-ffa450edef68