picojson::parse: use LC_NUMERIC = C

https://github.com/CleverRaven/Cataclysm-DDA/issues/3191 has risen again.
This was fixed with https://github.com/CleverRaven/Cataclysm-DDA/pull/3279 but never in master, which caused unexpected effects in 2e1dd07 

This sets LC_NUMERIC to sane values only while parsing.