Recent tidy-up broke builds with system double-conversion library

A recent clean-up of our double-conversion third-party code breaks building with system double-conversion libraries, which presume a #include <double-conversion/double-conversion.h> where the linked change removes the directory-layer from that path.
The upstream sources use #include "name.h" with no path component for their mutual inclusions, so building them requires an include path that has their directory in it; but for our code to include the export header as above, we need to give the compiler an include path with the last path-component omitted, so that the source can supply that last component.
Hopefully CMake can support using different include paths for different source files.