[CMake] CPack NSIS Silent install option

[J Decker]

Would be nice to be able to define a default option for uninstall
message box for silent install..


something like CPACK_NSIS_SILENT_UNINSTALL_DEFAULT

which could be set with
SET( CPACK_NSIS_SILENT_UNINSTALL_DEFAULT /SD IDYES )  # or IDNO or something



Modify Modules/NSIS.template.in  .. add silent uninstall default
before option list....


  MessageBox MB_YESNOCANCEL|MB_ICONEXCLAMATION \
  "@CPACK_NSIS_PACKAGE_NAME@ is already installed. $\n$\nDo you want
to uninstall the old version before installing the new one?" \
   @CPACK_NSIS_SILENT_UNINSTALL_DEFAULT@ IDYES uninst IDNO inst
  Abort