Make DCPoMatic compatible with ICU >= 75

Make DCPoMatic compatible with ICU >= 75

ICU >= 75 uses c++17 features and therefore requires compilation with
-std=c++17. However, this causes some namespace issues in
src/wx/file_picker_ctrl.cc and
src/wx/film_name_location_dialog.cc
between boost::optional, boost::filesystem and the corresponding
names from the std lib.

The patch fixes this namespace issues and adds a version check in
wscript to enable compilation with c++17, if icu >= 75 is detected.