I've learned that embedded copies of libraries are against Debian packaging policy. See here: https://wiki.debian.org/EmbeddedCopies
Therefore the best solution is to remove the embedded copy of the double-conversion library and add a dependency on libdouble-conversion-dev package. This should solve the FTBFS problem. I have asked quickfix upstream to remove the embedded copy. https://github.com/quickfix/quickfix/issues/393 If they prefer not to, then the Debian package maintainer can remove it to make this package comply with the policy above. -Alan
