Make it possible to link against system version of the library

`double-conversion` Haskell package copies the `double-conversion` C library into its source tree.
This works well enough for most use cases, but problems begin when you want to link your program with some other C library that depends on system installed `double-conversion` C library.

You end up with duplicate symbol definition errors like this:
```
double_conversion/lib/libdouble-conversion.a(double-conversion.o): multiple definition of 'double_conversion::DoubleToStringConverter::HandleSpecialValues(double, double_conversion::StringBuilder*) const'
```