message: Guard against empty Python_SITEARCH values
diff: @@ -18,6 +18,7 @@ function(string_strip_prefix in0 in1 outVar)
  set(lenMax ${len1})
  endif()
  set(idxNoPfx 0)
  + if(lenMax GREATER 0)
  foreach(len RANGE 1 ${lenMax})
  string(SUBSTRING "${in0}" 1 ${len} sub0)
  string(SUBSTRING "${in1}" 1 ${len} sub1)
  @@ -26,6 +27,7 @@ function(string_strip_prefix in0 in1 outVar)
  break()
  endif()
  endforeach()
  + endif()
  string(SUBSTRING "${in0}" ${idxNoPfx} ${len0} outTmp)
  set(${outVar} "${outTmp}" PARENT_SCOPE)
  endfunction()
  