Conditionally package DPInst

Conditionally package DPInst

Since DPInst.exe is not shipped with the Windows Driver Kit 10, an
environment variable must point to local copies. Make the inclusion of
DPInst conditional on DPINST_REDIST being defined and that path
existing. This simplifies building packages which do not require DPInst
for installation, and removes a required step to create a working build.

Signed-off-by: Owen Smith <owen.smith@citrix.com>