PROTON-413: Support for finding an installed proton that works on windows
Use in conjunction with commit associated with QPID-5103.
1. In proton: cmake -DCMAKE_INSTALL_PREFIX=P:/install ...
2.            devenv proton.sln /build "Debug|Win32" /project INSTALL
3. In qpid:   cmake -DPROTON_ROOT=P:/install ...
4.            devenv qpid-cpp.sln /build "Debug|Win32 /project INSTALL



git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1522678 13f79535-47bb-0310-9956-ffa450edef68