add build option new-namespace-installdir

[smoors]

better alternative solution for #4876

see also #4110

[smoors]

after discussion with @boegel i'll implement yet another solution that doesn't remove + re-create the install dir but instead removes all the contents of the install dir (using new function empty_dir), which has the same effect but may be a bit slower if it contains a lot of files.

[smoors]

solution using empty_dir #4932