Make it possible to link against system version of the library

[niteria]

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'

[enzoh]

This is a problem for me as well.

[enzoh]

I implemented a workaround for SpiderMonkey: https://github.com/dfinity/hs-double-conversion