Use an environment variable $(OpenSSLDir) to configure where VS should
find OpenSSL headers and libraries. Before running a build a user will
need to set the variable:

    SET OpenSSLDir=C:\OpenSSL-Win32

Change-Id: Ic0fccdfbce956161dddca7eb12bdec8ef9d3f29b
Signed-off-by: Thanh Ha <thanh.ha@eclipse.org>