Fix linking user applications against installed QuantLib with XAD support

When building QuantLib with XAD support using this repository, the examples and tests inside the repository work fine. However, when installing QuantLib with CMake into a prefix and building a third-party application against this installation is currently broken.

The idea is that, after installing XAD-enabled QuantLib into a prefix in CMake's search path, the following should work in CMake:

find_package(QuantLib REQUIRED)
add_executable(my_user_application source1.cpp ...)
target_link_libraries(my_user_application PRIVATE QuantLib::QuantLib)

In order to fix this, the install exports here should be modified to:

also install XAD itself
import this XAD dependency in the installed targets file
import (or define) the quantlib-xad interface target
amend the QuantLib target that is already created by QuantLib itself with the extra link library quantlib-xad to get all the settings right

auto-differentiation / QuantLib-Risks-Cpp

Fix linking user applications against installed QuantLib with XAD support #11