fix shapefiles encoding in demo for C++
fix shapefiles encoding in demo for C++

Mapnik builds without `boost_regex_icu` (see #3440) and the C++ demo does not run:

```
/mapnik# demo/c++/rundemo `mapnik-config --prefix`                                                                                                                                         
 running demo ... 
### std::exception: could not create converter for latin1
```
zerebubuth/mapnik,zerebubuth/mapnik,tomhughes/mapnik,pnorman/mapnik,CartoDB/mapnik,lightmare/mapnik,CartoDB/mapnik,mapnik/mapnik,tomhughes/mapnik,mapnik/mapnik,naturalatlas/mapnik,lightmare/mapnik,naturalatlas/mapnik,naturalatlas/mapnik,mapycz/mapnik,mapycz/mapnik,lightmare/mapnik,zerebubuth/mapnik,lightmare/mapnik,pnorman/mapnik,naturalatlas/mapnik,tomhughes/mapnik,pnorman/mapnik,tomhughes/mapnik,pnorman/mapnik,mapnik/mapnik,mapnik/mapnik,CartoDB/mapnik,mapycz/mapnik