SFINAE workaround for missing lconv support in Android (gcc + libstdc++)
See: 2fd6c0b245
Discussion: https://github.com/nlohmann/json/issues/638#issuecomment-314079985
Note: Clang + libc++ should work (where available)
Returns the thousands separator for the current locale. On android (gcc/libstdc++) the lconv structure is stubbed using an empty structure. The test is for the size only, not for the presense of thousands_sep in std::lconv, because if one would add thousands_sep at some point, the size of structure would be at least sizeof(char*)
Ditto for decimal_point.