[Tile-serving] [openstreetmap/mod_tile] Fix compilation on CentOS7 7.2. (#146)

[Tom Hughes]

I suspect this can be simplified. The code defining `mapnik::parameters` in 2.2.x is:

```
typedef boost::variant<value_null,value_integer,value_double,std::string> value_
holder;
typedef std::map<std::string, value_holder> param_map;
class MAPNIK_DECL parameters : public param_map
```

and in master:

```
using value_holder_base = util::variant<value_null,value_integer,value_double,st
d::string,value_bool>;
struct value_holder : value_holder_base
using param_map = std::map<std::string, value_holder>;
class MAPNIK_DECL parameters : public param_map
```

So the only change is the insertion of a wrapper around the variant, but that has a perfect forward for construction so it's likely that the other change, from boost::variant to mapbox::variant is the cause.

But in any case `std::string` is a valid member of the variant in both cases, so the only question is whether it can implicitly convert from a `char *` or not, and the explicit conversion should always be OK without any version check.



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