!macro !defineifexist _VAR_NAME _FILE_NAME
  !tempfile _TEMPFILE
  !ifdef NSIS_WIN32_MAKENSIS
    ; Windows - cmd.exe
    !system 'if exist "${_FILE_NAME}" echo !define ${_VAR_NAME} > "${_TEMPFILE}"'
  !else
    ; Posix - sh
    !system 'if [ -e "${_FILE_NAME}" ]; then echo "!define ${_VAR_NAME}" > "${_TEMPFILE}"; fi'
  !endif
  !include '${_TEMPFILE}'
  !delfile '${_TEMPFILE}'
  !undef _TEMPFILE
!macroend
!define !defineifexist "!insertmacro !defineifexist"